Suppress extended Clang warning -Wmicrosoft-cast

Disable the warning until we fix the cast in

  src\third_party\smhasher\src\PMurHash.c

Bug:  1054220

Change-Id: Ib08664366f22da17d5cb1a5d801e3317de64a27a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2065736Reviewed-by: Reid Kleckner <rnk@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743353}

build/config/compiler/BUILD.gn

@@ -1545,6 +1545,14 @@ config("default_warnings") {
             # TODO(https://crbug.com/1050281): Clean up, enable.
             "-Wno-non-c-typedef-for-linkage",
           ]
+
+          cflags_c += [
+            # TODO(https://crbug.com/1054220): This used to be a C++-only
+            # warning, but it was extended to .C files as well, where it
+            # caught a cast of pointer to long in third-party that isn't
+            # valid on Windows, which is LLP64.
+            "-Wno-microsoft-cast",
+          ]
         }
 
         cflags_c += [