Build the Clang plugin on Windows.
This also includes several fixes for the plugin logic on Windows:
- The banned directory check normalizes path separators.
- The banned directory check now returns true for files that are
considered system headers.
- Diagnostics from the plugin are always reported as warnings.
BUG=467287
R=thakis@chromium.org
Committed: https://chromium.googlesource.com/chromium/src/+/
26ba172e8543b9029f320600f3aeced40028ff20
Review URL: https://codereview.chromium.org/
1072203002
Cr-Commit-Position: refs/heads/master@{#325365}