Roll clang 283753:284979.

Ran `tools/clang/scripts/upload_revision.py 284979`.

BUG=656667

Review-Url: https://codereview.chromium.org/2443963002
Cr-Commit-Position: refs/heads/master@{#427269}

tools/clang/scripts/update.py

@@ -27,7 +27,7 @@ import zipfile
 # Do NOT CHANGE this if you don't know what you're doing -- see
 # https://chromium.googlesource.com/chromium/src/+/master/docs/updating_clang.md
 # Reverting problematic clang rolls is safe, though.
-CLANG_REVISION = '283753'
+CLANG_REVISION = '284979'
 
 use_head_revision = 'LLVM_FORCE_HEAD_REVISION' in os.environ
 if use_head_revision: