Preparation for actual SPIR-V output instead of GLSL.

Bug: angleproject:4889
Change-Id: Ic279b23d3a817bd5dca66a844905378207afdbac
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2721194
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>