[Win32] Don't warn on #pragma optimize (#563)

Disable this warning when building with clang on Windows, which has no
support for #pragma optimize.

See: https://crbug.com/505314#c13

Issue: https://github.com/flutter/flutter/issues/59199

build/config/compiler/BUILD.gn

diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index 999c6fb..8282fb4 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -605,6 +605,12 @@
       "-Wno-nonportable-system-include-path",
     ]
   }
+  if (is_clang) {
+    default_warning_flags += [
+      # No support for #pragma optimize. https://crbug.com/505314#c13
+      "-Wno-ignored-pragma-optimize",
+    ]
+  }
 } else {
   # Common GCC warning setup.
   default_warning_flags += [