Add support for multisampled sampling

This CL adds support for multisampled sampling on the D3D12 backend.
This was already working on other backends. It also adds tests that all
of the sample locations are correct.

Bug: dawn:431
Change-Id: I6849e5e2d708ad4824e6db2665d668d43a4ef5ea
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/23245
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

src/common/BUILD.gn

@@ -96,15 +96,16 @@ config("dawn_internal") {
       "-Wc++11-narrowing",
       "-Wdeprecated-copy",
       "-Wextra-semi-stmt",
+      "-Wimplicit-fallthrough",
       "-Winconsistent-missing-destructor-override",
       "-Winvalid-offsetof",
       "-Wmissing-field-initializers",
+      "-Wnon-c-typedef-for-linkage",
       "-Wpessimizing-move",
       "-Wreturn-std-move-in-c++11",
       "-Wshadow-field",
       "-Wstrict-prototypes",
       "-Wtautological-unsigned-zero-compare",
-      "-Wnon-c-typedef-for-linkage",
     ]
 
     if (is_win) {

src/common/Compiler.h

@@ -107,4 +107,10 @@ extern void __cdecl __debugbreak(void);
 #    define DAWN_FORCE_INLINE inline
 #endif
 
+#if defined(__clang__)
+#    define DAWN_FALLTHROUGH [[clang::fallthrough]]
+#else
+#    define DAWN_FALLTHROUGH
+#endif
+
 #endif  // COMMON_COMPILER_H_