This is the first change in a series to generate SPIR-V directly from
the translator's AST, instead of the generating text GLSL and feeding it
to glslang.

This change implements the majority of the work needed to map AST types
to SPIR-V types, and declare types and interface variables in SPIR-V.
Additionally, it lays the infrastructure to conditionally enabling this
path in end2end tests.

No tests are currently enabled as the change doesn't actually generate
code for function bodies yet.

Bug: angleproject:4889
Change-Id: Iacb28b6907fd48c50e4cc5a0e7ad72f6eed241d4
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2889603
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>