win: Re-enable libc++ on Windows.

We now have a natvis file for libc++ and fixed a bug in lld-link that
made it not embed natvis files correctly if there were more than two,
so now this switch should no longer negatively affect debugging.

Bug: 801780
Change-Id: Iab1b3f8b3e3bbbda3abdf08156287820392d7946
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1594345
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#656840}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 4b298260f5327c09983083055a5775d5ce719cc1

config/c++/c++.gni

@@ -12,6 +12,7 @@ declare_args() {
   # is not supported.
   use_custom_libcxx =
       is_fuchsia || is_android || is_mac || (is_ios && !use_xcode_clang) ||
+      (is_win && is_clang) ||
       (is_linux &&
        (!is_chromeos || default_toolchain != "//build/toolchain/cros:target"))