This CL adds generation of Struct types to the SPIR-V writer. Bug: tint:5 Change-Id: Ibcabf7b1a688026297de682f4825d5195d8007d2 Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/17701 Reviewed-by: David Neto <dneto@google.com>