Use clang versions of SK_RESTRICT and SK_WARN_UNUSED_RESULT on Windows

Bug: skia:
Change-Id: I63bc32c7bba294d8698d17e1f178ca0eb0a84ddf
Reviewed-on: https://skia-review.googlesource.com/c/191295
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: Brian Osman <brianosman@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
diff --git a/include/core/SkPreConfig.h b/include/core/SkPreConfig.h
index 1f83c14..923d965 100644
--- a/include/core/SkPreConfig.h
+++ b/include/core/SkPreConfig.h
@@ -44,7 +44,7 @@
 
 //////////////////////////////////////////////////////////////////////
 
-#ifdef SK_BUILD_FOR_WIN
+#if defined(SK_BUILD_FOR_WIN) && !defined(__clang__)
     #if !defined(SK_RESTRICT)
         #define SK_RESTRICT __restrict
     #endif