Suppress new Clang warning -Wbuiltin-assume-aligned-alignment

Bug: 1016945
Change-Id: Ide11a3e8f26ed447668f1497daf4392d0ba0d605
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1879889
Auto-Submit: Hans Wennborg <hans@chromium.org>
Reviewed-by: Reid Kleckner <rnk@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709270}

build/config/compiler/BUILD.gn

@@ -1539,6 +1539,9 @@ config("default_warnings") {
           cflags += [
             # TODO(https://crbug.com/1016947) Clean up, enable.
             "-Wno-bitwise-conditional-parentheses",
+
+            # TODO(https://crbug.com/1016945) Clean up, enable.
+            "-Wno-builtin-assume-aligned-alignment",
           ]
         }
       }