Atomic counters are not supported by Vulkan. Most are
already converted by the RewriteAtomicCounters traversal,
but that is only invoked when atomic counters are active.

This CL introduces another pass that removes any atomic
counter builtin that was not handled by the previous pass.
It also will assert if it sees any atomic counters active,
thus ensuring it is only used when needed.

Test: KHR-GLES31.core.compute_shader.shared-struct
Test: angle_end2end_tests.exe --gtest_filter="*AtomicCounter*"
Bug: angleproject:4189
Bug: b:150310216
Change-Id: I61d10e954886dc94fede8b344f5a0ede3b689adb
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2095688
Commit-Queue: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>