document COMPILER(GCC) and COMPILER(MSVC) a little better

third_party/WebKit/Source/wtf/Compiler.h

 /*
  * Copyright (C) 2011, 2012 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 24 matching lines @@
 /* COMPILER_QUIRK() - whether the compiler being used to build the project requires a given quirk. */
 #define COMPILER_QUIRK(WTF_COMPILER_QUIRK) (defined WTF_COMPILER_QUIRK_##WTF_COMPILER_QUIRK  && WTF_COMPILER_QUIRK_##WTF_COMPILER_QUIRK)
 
 /* ==== COMPILER() - the compiler being used to build the project ==== */
 
 /* COMPILER(CLANG) - Clang  */
 #if defined(__clang__)
 #define WTF_COMPILER_CLANG 1
 #endif
 
-/* COMPILER(MSVC) - Microsoft Visual C++ */
+/* COMPILER(MSVC) - Microsoft Visual C++ (and Clang when compiling for Windows). */
 #if defined(_MSC_VER)
 #define WTF_COMPILER_MSVC 1
 #endif
 
-/* COMPILER(GCC) - GNU Compiler Collection */
+/* COMPILER(GCC) - GNU Compiler Collection (and Clang when compiling for platforms other than Windows). */
 #if defined(__GNUC__)
 #define WTF_COMPILER_GCC 1
 #define GCC_VERSION (__GNUC__ * 10000 + __GNUC_MINOR__ * 100 + __GNUC_PATCHLEVEL__)
 #define GCC_VERSION_AT_LEAST(major, minor, patch) (GCC_VERSION >= (major * 10000 + minor * 100 + patch))
 #else
 /* Define this for !GCC compilers, just so we can write things like GCC_VERSION_AT_LEAST(4, 1, 0). */
 #define GCC_VERSION_AT_LEAST(major, minor, patch) 0
 #endif
 
 /* ==== Compiler features ==== */
@@ skipping 101 matching lines @@
 /* NO_SANITIZE_UNRELATED_CAST - Disable runtime checks related to casts between
  * unrelated objects (-fsanitize=cfi-unrelated-cast or -fsanitize=vptr). */
 
 #if COMPILER(CLANG)
 #define NO_SANITIZE_UNRELATED_CAST __attribute__((no_sanitize("cfi-unrelated-cast", "vptr")))
 #else
 #define NO_SANITIZE_UNRELATED_CAST
 #endif
 
 #endif /* WTF_Compiler_h */