Adds support for the new shader IO syntax by processing entry
parameters and pushing them to function-scope variables as necessary.
Module-scope variables are still supported for now.
Fixed: tint:731
Change-Id: I36d7ce4e3a990b6323292cb7c685af37187d6fda
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/48960
Auto-Submit: James Price <jrprice@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>