Most GLSL built-ins map directly to some SPIR-V operation.  texture*()
and image*() built-ins are not implemented as well as a handful of other
built-ins that require special-handling.

Bug: angleproject:4889
Change-Id: I72abfb4692c3d66a3c8be43ce44ba6808efe6255
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2971646
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>