This change introduces a SPIR-V transformer that modifies shader
interface variable decorations directly in SPIR-V instead of
manipulating the input GLSL.  Currently, descriptor set and binding
indices are set by the transformer.

The shader translator outputs arbitrary set and binding indices.  Once
compiled by glslang, the SPIR-V transformer modifies these decorations.
The ultimate goal is to be able to modify the SPIR-V again when program
pipeline objects decide a different set/binding is necessary.

Bug: angleproject:3394
Change-Id: If358265a72bf1fe9f5676562b39a632cb2e05dc4
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2001477Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>