Reland "Restrict where 'binding' and 'set' can appear"

This is a reland of 9372ef0228be737c562122f93de057c11d170cfe

Original change's description:
> Restrict where 'binding' and 'set' can appear
>
> In SPIRV, these are an error when applied to struct members. Some of our
> tests were triggering that because we had free-floating uniforms
> decorated this way (and we coalesce those into members of an interface
> block).
>
> Now, we only allow those layout qualifiers on variable types that will
> remain top-level constructs in the back-end.
>
> Bug: skia:12670
> Change-Id: I73e69cecf6237a1c1180ad38d9b5d52ea80316fb
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/474218
> Commit-Queue: Brian Osman <brianosman@google.com>
> Reviewed-by: Ethan Nicholas <ethannicholas@google.com>

Bug: skia:12670
Change-Id: I01c0323bba7ce0bddea5f9fb907e2b60e6b812d2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/475156
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Brian Osman <brianosman@google.com>
17 files changed