Reenable libc++ on Win/libfuzzer

This is a partial revert of [1].  Keep disabling libc++ on Win/cl.exe, but
reenable it with Win/libfuzzer since the timeout issues should (hopefully) be
fixed after [2].

[1] https://chromium-review.googlesource.com/c/chromium/src/+/1527776
[2] https://chromium.googlesource.com/chromium/src.git/+/f05d184130d651fd7ab3f5712aaeaf53a0cf0815

BUG=942939
R=thakis

Change-Id: Iad160248b0d6fe631ac71719ba6b1e23728c4fa2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1533183
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642788}
1 file changed