GLSL contains a large number of built-in texture* and image* functions,
but these map to only a handful of SPIR-V instructions.  The bulk of the
work to map these is to extract the arguments from the built-ins based
on their ordinal position.

Bug: angleproject:4889
Change-Id: I760d986bd9171ddde35f9f046c549ca53252df17
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2992980Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>