Add experimental pass manager by default in clang

CMakeLists.txt

@@ -300,6 +300,11 @@ if (COMPILER_CLANG)
         option(ENABLE_THINLTO "Clang-specific link time optimization" ON)
     endif()
 
+    # Set new experimental pass manager, it's a performance, build time and binary size win.
+    # Can be removed after https://reviews.llvm.org/D66490 merged and released to at least two versions of clang.
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fexperimental-new-pass-manager")
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fexperimental-new-pass-manager")
+
     # We cannot afford to use LTO when compiling unit tests, and it's not enough
     # to only supply -fno-lto at the final linking stage. So we disable it
     # completely.