And remove a whole load of const_cast hackery.
Semantic nodes may contain internally mutable fields (although only ever modified during resolving), so these are always passed by `const` pointer.
While all AST nodes are internally immutable, we have decided that pointers to AST nodes should also be marked `const`, for consistency.
There's still a collection of const_cast calls in the Resolver. These will be fixed up in a later change.
Bug: tint:745
Change-Id: I046309b8e586772605fc0fe6b2d27f28806d40ef
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/66606
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@chromium.org>
Reviewed-by: David Neto <dneto@google.com>
8648120bbe