[clang] Make update.py verify that the expected and real versions match

To fix problems when building with a new version of update.py without
having run 'gclient sync', which could e.g. lead to PCH files built with
the wrong clang version being left in the tree (see bug).

Bug: 608105
Change-Id: Ia5b70506be3f6a89d17742b6b2e0bf3271585d31
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3847019


Commit-Queue: Hans Wennborg <hans@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1038229}