Also create it in wgsl parser. With this, we now create all the AST type
nodes in the wgsl parser.
Bug: tint:724
Change-Id: I575c9eb0ffbd60c3e7aca0b00227d9833ef65d58
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/48962
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>