Need to declare use_xcode_clang when building outside chromium.

Bug: angleproject:5440
Change-Id: I4d622c1b6085d89b69c828b2c965d22c1f806f45
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2579750Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: James Darpinian <jdarpinian@chromium.org>