clang/win: Make diagnostics colored if ansicon is around at runhooks time.

BUG=82385
TEST=Run gyp or gn with ansicon active, build. Diagnostics should be colored
in cmd.exe.

Review URL: https://codereview.chromium.org/1194823004

Cr-Commit-Position: refs/heads/master@{#335394}
3 files changed