Disable libc++ on Windows/cl.exe and Windows/libfuzzer

Windows/libc++ has been in the tree for about 1.5 days now and fortunately the
only real failures are timeouts.  Rather than reverting and disabling libc++
everywhere, instead selectively disable it for the configurations that are
having issues.

Specifically, the V8 cl.exe build has broken, so disable libc++ there.  Several
Windows fuzzers are timing out, so temporarily disable libc++ for libfuzzer.

BUG=801780,942915,942939
R=thakis

Change-Id: I9dd0d4c435f499c8e7cf05cdf14bb0fc2829fa15
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1527776
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#641507}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 64b50e0ea351d597c2a5f882283888756e82d91d

config/c++/c++.gni

@@ -10,8 +10,10 @@ declare_args() {
   # standard library support.
   # Don't check in changes that set this to false for more platforms; doing so
   # is not supported.
+  # TODO(https://crbug.com/942939): Re-enable libc++ on Windows with libfuzzer.
   use_custom_libcxx =
-      is_fuchsia || is_android || is_mac || is_win ||
+      is_fuchsia || is_android || is_mac ||
+      (is_win && is_clang && !use_libfuzzer) ||
       (is_linux &&
        (!is_chromeos || default_toolchain != "//build/toolchain/cros:target"))