Add a return type decoration list field to ast::Function.
Bug: tint:513
Change-Id: I41c1087f21a87731eb48ec7642997da5ae7f2baa
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/44601
Commit-Queue: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>