Merge Windows and Posix libc++ configs

BUG=801780
R=thakis

Change-Id: I9ce7870d4205aedb02ca49a2483b7275b1c4b757
Reviewed-on: https://chromium-review.googlesource.com/c/1461335
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631075}
5 files changed