Last time this was tracked in  Issue 869850 

Patch: https://chromium-review.googlesource.com/c/chromium/src/+/1412828

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/70d9d4df426e299d1f7d68d8d6215594afe21a6d

commit 70d9d4df426e299d1f7d68d8d6215594afe21a6d
Author: Hans Wennborg <hans@chromium.org>
Date: Wed Jan 16 11:09:58 2019

Clang: the tip-of-tree version is now 9

TBR=thakis

Bug: 922386
Change-Id: I14856f18ea5025fd9f1fd813e3fe87bdb787a656
Reviewed-on: https://chromium-review.googlesource.com/c/1412828
Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623192}
[modify] https://crrev.com/70d9d4df426e299d1f7d68d8d6215594afe21a6d/build/toolchain/toolchain.gni
[modify] https://crrev.com/70d9d4df426e299d1f7d68d8d6215594afe21a6d/tools/clang/scripts/update.py

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d8927fce44de956fae4c310fa13d36be0cfc7378

commit d8927fce44de956fae4c310fa13d36be0cfc7378
Author: Reid Kleckner <rnk@google.com>
Date: Sat Jan 19 00:00:41 2019

Roll clang 350768:351477.

Update VERSION from 8.0.0 to 9.0.0 in update.py and toolchain.gni.

Add -fsplit-lto-unit to account for a change in behavior in upstream
ThinLTO.

Bug:  922384 , 922386
Change-Id: Ic9dbf0a9f4d8c50756b8bdb01c75782ba387bdca
Reviewed-on: https://chromium-review.googlesource.com/c/1418440
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Peter Collingbourne <pcc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624366}
[modify] https://crrev.com/d8927fce44de956fae4c310fa13d36be0cfc7378/build/config/compiler/BUILD.gn
[modify] https://crrev.com/d8927fce44de956fae4c310fa13d36be0cfc7378/build/toolchain/toolchain.gni
[modify] https://crrev.com/d8927fce44de956fae4c310fa13d36be0cfc7378/tools/clang/scripts/update.py