[tint][ir][val] Enforce rules around scope for function vars

- Adds a check that function address space vars are only declared in a
  function scope
- Adds a check that non-function address space are not declared in a
  function scope
- Add a capability to allow MSL specific IR to have private address
  space vars declared in a function scope.
- Bunch of test fixes/updates

Fixes: 381109471
Fixes: 381109773
Change-Id: I874930a858777c43a0c26354c7d888ffeab9056c
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/216957
Auto-Submit: Ryan Harrison <rharrison@chromium.org>
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Reviewed-by: James Price <jrprice@google.com>
Commit-Queue: dan sinclair <dsinclair@chromium.org>