Revert "Roll clang 337439:338452."

This reverts commit cc14e55.

Reason for revert:
The new version of clang appears to have regressed -O0 code size significantly (https://crbug.com/870907)

Original change's description:
> Roll clang 337439:338452.
>
> Bug: 866225
> Change-Id: I11d97d9a97432a1abb97e69a1395948b9d3e19fe
> Reviewed-on: https://chromium-review.googlesource.com/1157290
> Commit-Queue: Reid Kleckner <rnk@chromium.org>
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#579922}

TBR=thakis@chromium.org,hans@chromium.org,rnk@chromium.org
NOTRY=true

Bug: 866225, 870907
Change-Id: I39ac706d7fc4191d4519f1692f927788a072a081
Reviewed-on: https://chromium-review.googlesource.com/1170006
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Reid Kleckner <rnk@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582026}

tools/clang/scripts/update.py

@@ -27,7 +27,7 @@
 # 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 = '338452'
+CLANG_REVISION = '337439'
 
 use_head_revision = bool(os.environ.get('LLVM_FORCE_HEAD_REVISION', '0')
                          in ('1', 'YES'))