This change enables direct SPIR-V generation for compute shaders and
enables a few tests.  A handful of built-in functions are translated to
support said tests.

Bug: angleproject:4889
Change-Id: I8fd6dc50ff31559a738ba680a993fb197e29fcf9
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2939330
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>