MSL has a limit on the number of threadgroup memory arguments, so use
a struct to support an arbitrary number of workgroup variables.
This commit introduces a `State` object to this transform, which is
used to track which structs have been cloned eagerly, in order to
avoid duplicating them.
Bug: tint:938
Change-Id: Ia467db186e176a08f160455eab5fd3b3662f56b8
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/65360
Auto-Submit: James Price <jrprice@google.com>
Kokoro: James Price <jrprice@google.com>
Commit-Queue: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>