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>
17 files changed