This CL starts adding global variable output to the SPIR-V writer. This
does not handle constants or initializers at this point.
Bug: tint:5
Change-Id: Id06533b2ec1f61feadf66f3e43a484f6f3765546
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/17922
Reviewed-by: David Neto <dneto@google.com>