This will be used to validate layout rules, as well as preventing
illegal types from being used in a uniform / storage buffer.
Also: Cleanup logic around VariableDeclStatement
This was spread across 3 places, entirely unnecessarily.
Bug: tint:643
Change-Id: I9d309c3a5dfb5676984f49ce51763a97bcac93bb
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/45125
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: James Price <jrprice@google.com>
Reviewed-by: David Neto <dneto@google.com>