Downgrade double-promotion from error to warning

https://github.com/harfbuzz/harfbuzz/issues/1740

src/hb.hh

@@ -66,7 +66,6 @@
 #pragma GCC diagnostic error   "-Wcast-align"
 #pragma GCC diagnostic error   "-Wcast-function-type"
 #pragma GCC diagnostic error   "-Wdelete-non-virtual-dtor"
-#pragma GCC diagnostic error   "-Wdouble-promotion"
 #pragma GCC diagnostic error   "-Wextra-semi-stmt"
 #pragma GCC diagnostic error   "-Wformat-security"
 #pragma GCC diagnostic error   "-Wimplicit-function-declaration"
@@ -99,6 +98,7 @@
 #pragma GCC diagnostic warning "-Wbuiltin-macro-redefined"
 #pragma GCC diagnostic warning "-Wdeprecated"
 #pragma GCC diagnostic warning "-Wdisabled-optimization"
+#pragma GCC diagnostic warning "-Wdouble-promotion"
 #pragma GCC diagnostic warning "-Wformat=2"
 #pragma GCC diagnostic warning "-Wignored-pragma-optimize"
 #pragma GCC diagnostic warning "-Wlogical-op"