New traces are hitting string literal limits with MSVC. Since we
don't use MSVC or GCC for trace development or testing, this CL
disables trace compilation for non-clang setups.

Bug: b/181794064
Bug: angleproject:6009
Change-Id: I0377792a049153a5505ecda4346a3cec57c12e9e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2922682Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Cody Northrop <cnorthrop@google.com>