Revert of Build the Clang plugin on Windows.

Revert of Build the Clang plugin on Windows. (patchset #6 id:100001 of https://codereview.chromium.org/1072203002/)

Reason for revert:
Not all the Windows bots have CMake 3.1+.

Original issue's description:
> 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

TBR=hans@chromium.org,thakis@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=467287
Committed: https://crrev.com/627a8417950a65559f58c756a902fdfff7de5dbf
Cr-Commit-Position: refs/heads/master@{#324795}

[dcheng, 2015-04-12 07:33:00]

Created Revert of Build the Clang plugin on Windows.

[commit-bot: I haz the power, 2015-04-12 07:33:04]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1070353003/1

[commit-bot: I haz the power, 2015-04-12 07:33:45]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-12 07:35:41]

Patchset 1 (id:??) landed as
https://crrev.com/627a8417950a65559f58c756a902fdfff7de5dbf
Cr-Commit-Position: refs/heads/master@{#324795}

[Nico, 2015-04-13 00:07:36]

Looks like this revert wasn't enough, this is still causing trouble on these two
bots:

http://build.chromium.org/p/chromium.fyi/builders/CrWinClangLLD
http://build.chromium.org/p/chromium.fyi/builders/CrWinAsan

(I think they're the same physical machine; maybe that's the only without cmake
3.1)