Suppress -Wunguarded-availability in MetalBackend.h

Chromium will use this header and currently uses a minimum SDK version
without Metal, so it gets a compilation warning on MTLDevice. There is
no good way to guard availability in headers, so just suppress the
warning in the whole header and push the warning to usage of types in
client code.

BUG=chromium:938895

Change-Id: Id8ec224825c39a28a404be216556780713ecaf15
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/5440
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/include/dawn_native/MetalBackend.h

@@ -18,6 +18,13 @@
 #include <dawn/dawn_wsi.h>
 #include <dawn_native/DawnNative.h>
 
+// The specifics of the Metal backend expose types in function signatures that might not be
+// available in dependent's minimum supported SDK version. Suppress all availability errors using
+// clang's pragmas. Dependents using the types without guarded availability will still get errors
+// when using the types.
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wunguarded-availability"
+
 struct __IOSurface;
 typedef __IOSurface* IOSurfaceRef;
 
@@ -38,4 +45,6 @@ namespace dawn_native { namespace metal {
 }}      // namespace dawn_native::metal
 #endif  // __OBJC__
 
+#pragma clang diagnostic pop
+
 #endif  // DAWNNATIVE_METALBACKEND_H_