Revert "Roll clang 342523:343189."

This reverts commit 7c58d162.

Reason for revert:
This broke win-asan, see 2nd bug.

Original change's description:
> Roll clang 342523:343189.
> 
> Bug: 888476
> Change-Id: Iccb39603a10261d1bdf3fcb26b008bb02fc371e6
> Reviewed-on: https://chromium-review.googlesource.com/1245465
> Commit-Queue: Nico Weber <thakis@chromium.org>
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#594765}

TBR=thakis@chromium.org,hans@chromium.org

Change-Id: I003bbfca7a542fe18c31a679ea58cf1da2c3fdb3
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 888476, 890310
Reviewed-on: https://chromium-review.googlesource.com/1251606Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595081}

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 = '343189'
+CLANG_REVISION = '342523'
 
 use_head_revision = bool(os.environ.get('LLVM_FORCE_HEAD_REVISION', '0')
                          in ('1', 'YES'))