This Cl extracts the variable stack from the SPIR-V builder and makes it
available as a general class.
Change-Id: I3505d7cc4ec34f78bbc3360b064fa99479fd97d6
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/18701
Reviewed-by: David Neto <dneto@google.com>