A bug in clang is preventing the use of link time optimization.

Bug: angleproject:2892
Change-Id: Ibe845e82f0db48233f8590189163fd8a5a8df634
Reviewed-on: https://chromium-review.googlesource.com/c/1263905Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>