Vulkan allows them, but WGSL does not.
A duplicate decoration is purely redundant.
A parameterized decoratio with different parameterization is
an inconsistency and a semantic error, at least for currently defined
SPIR-V decorations.
So for each target, only take the first decoration of each kind.
Fixed: tint:1337
Change-Id: I6ed5c39cf2e213c695cb8217ed1b97814da3db56
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/72500
Reviewed-by: Ben Clayton <bclayton@google.com>
Auto-Submit: David Neto <dneto@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: James Price <jrprice@google.com>
Commit-Queue: James Price <jrprice@google.com>