__has_include seems to cause problems with goma builds.
Instead, detect iOS vs MacOS using TargetConditionals.h. Also use
plain C++ instead of Objective-C++ when possible.

Bug: angleproject:3439
Bug: 1015591
Change-Id: I816624e0cdc54ad3a18d3891b4efecf6fe640574
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1894243Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: James Darpinian <jdarpinian@chromium.org>