Revert "Update compiler to Clang 10. (#13177)" (#13181)

This reverts commit 6fd50b36.

Reason: this seems to break our LUCI build https://ci.chromium.org/p/flutter/g/engine/console

Specifically, the following new issues were found that need to addressed before attempting
this roll again.

1. More symbols are being exported out of libflutter.so on Android https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket.appspot.com/8899411714657886224/+/steps/Verify_exported_symbols_on_release_binaries/0/stdout

2. ___isOSVersionAtLeast is not present on iOS https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket.appspot.com/8899411714401395808/+/steps/build_ios_debug_sim/0/stdout

DEPS

@@ -137,7 +137,7 @@ allowed_hosts = [
 ]
 
 deps = {
-  'src': 'https://github.com/flutter/buildroot.git' + '@' + 'b35cf60d674acec0bf0f1b58f3ee044fed449270',
+  'src': 'https://github.com/flutter/buildroot.git' + '@' + '9c893948fd2413b32669b5a9261a1b6a487b8f9b',
 
    # Fuchsia compatibility
    #
@@ -528,7 +528,7 @@ deps = {
     'packages': [
       {
         'package': 'fuchsia/clang/${{platform}}',
-        'version': 'git_revision:fce11c6904c888e6d39f71e03806a540852dec41'
+        'version': 'git_revision:de39621f0f03f20633bdfa50bde97a3908bf6e98'
       }
     ],
     'condition': 'host_os == "mac" or host_os == "linux"',

shell/common/input_events_unittests.cc

@@ -183,54 +183,54 @@ TEST_F(ShellTest, DelayAtMostOneEventForFasterThanVSyncInputEvents) {
 TEST_F(ShellTest, HandlesActualIphoneXsInputEvents) {
   // Actual delivery times measured on iPhone Xs, in the unit of frame_time
   // (16.67ms for 60Hz).
-  double iphone_xs_times[] = {0.15,
-                              1.0773046874999999,
-                              2.1738720703124996,
-                              3.0579052734374996,
-                              4.0890087890624995,
-                              5.0952685546875,
-                              6.1251708984375,
-                              7.1253076171875,
-                              8.125927734374999,
-                              9.37248046875,
-                              10.133950195312499,
-                              11.161201171875,
-                              12.226992187499999,
-                              13.1443798828125,
-                              14.440327148437499,
-                              15.091684570312498,
-                              16.138681640625,
-                              17.126469726562497,
-                              18.1592431640625,
-                              19.371372070312496,
-                              20.033774414062496,
-                              21.021782226562497,
-                              22.070053710937497,
-                              23.325541992187496,
-                              24.119648437499997,
-                              25.084262695312496,
-                              26.077866210937497,
-                              27.036547851562496,
-                              28.035073242187497,
-                              29.081411132812498,
-                              30.066064453124998,
-                              31.089360351562497,
-                              32.086142578125,
-                              33.4618798828125,
-                              34.14697265624999,
-                              35.0513525390625,
-                              36.136025390624994,
-                              37.1618408203125,
-                              38.144472656249995,
-                              39.201123046875,
-                              40.4339501953125,
-                              41.1552099609375,
-                              42.102128906249995,
-                              43.0426318359375,
-                              44.070131835937495,
-                              45.08862304687499,
-                              46.091469726562494};
-  int n = sizeof(iphone_xs_times) / sizeof(iphone_xs_times[0]);
+  constexpr double iphone_xs_times[] = {0.15,
+                                        1.0773046874999999,
+                                        2.1738720703124996,
+                                        3.0579052734374996,
+                                        4.0890087890624995,
+                                        5.0952685546875,
+                                        6.1251708984375,
+                                        7.1253076171875,
+                                        8.125927734374999,
+                                        9.37248046875,
+                                        10.133950195312499,
+                                        11.161201171875,
+                                        12.226992187499999,
+                                        13.1443798828125,
+                                        14.440327148437499,
+                                        15.091684570312498,
+                                        16.138681640625,
+                                        17.126469726562497,
+                                        18.1592431640625,
+                                        19.371372070312496,
+                                        20.033774414062496,
+                                        21.021782226562497,
+                                        22.070053710937497,
+                                        23.325541992187496,
+                                        24.119648437499997,
+                                        25.084262695312496,
+                                        26.077866210937497,
+                                        27.036547851562496,
+                                        28.035073242187497,
+                                        29.081411132812498,
+                                        30.066064453124998,
+                                        31.089360351562497,
+                                        32.086142578125,
+                                        33.4618798828125,
+                                        34.14697265624999,
+                                        35.0513525390625,
+                                        36.136025390624994,
+                                        37.1618408203125,
+                                        38.144472656249995,
+                                        39.201123046875,
+                                        40.4339501953125,
+                                        41.1552099609375,
+                                        42.102128906249995,
+                                        43.0426318359375,
+                                        44.070131835937495,
+                                        45.08862304687499,
+                                        46.091469726562494};
+  constexpr int n = sizeof(iphone_xs_times) / sizeof(iphone_xs_times[0]);
   // We don't use `constexpr int frame_time` here because MSVC doesn't handle
   // it well with lambda capture.
   UnitlessTime frame_time = 10000;