This makes shader compiler support the new basic type 'atomic_uint'
and validate its layout qualifiers properly.

BUG=angleproject:1729
TEST=angle_unittests:AtomicCounterTest
     angle_deqp_gles31_tests:dEQP-GLES31.functional.atomic_counter.layout.invalid*
     angle_deqp_gles31_tests:dEQP-GLES31.functional.debug.negative_coverage.*.atomic*

Change-Id: Ia237eadf6ea72314f436a0abbb93a05598e71eba
Reviewed-on: https://chromium-review.googlesource.com/500088
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>