Pull the mutable semantic field from ast::Variable and into a new semantic::Variable node.
Have the TypeDeterminer create these semantic::Variable nodes.
Bug: tint:390
Change-Id: Ia13f5e7b065941ed66ea5a86c6ccb288071feff3
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/40063
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: David Neto <dneto@google.com>