GitLab

* win: Members of base classes now should show up in debugger.
* win: Debugger shouldn't show funny highlights anymore due to
  debug info no longer including column information.  (we still
  force this on if sanitizers are used, mostly for clusterfuzz.
  maybe we want to make this toggleable independent of sanitizers
  at some point)
* win: -Wextern-initializer no longer warns on midl-generated code
* win: clang-cl now accepts /source-encoding:utf-8 and friends
  (utf-8 was the source enconding in clang-cl before already, but
  now we don't warn on an explicit flag requesting this)
* all platforms: Three plugin checks are now on-by-default,
  remove flags for these (see
    https://codereview.chromium.org/2267713003
    https://codereview.chromium.org/2268203002
    https://codereview.chromium.org/2265093002
  )
* win: clang-cl's /Brepro now does what it's supposed to do
* win: clang-cl now emits absolute paths in diagnostics, by
  popular request.

Ran `tools/clang/scripts/upload_revision.py 280106`.

BUG=640254,637456,636109,636091,636099

Review-Url: https://codereview.chromium.org/2292173002
Cr-Commit-Position: refs/heads/master@{#415563}