Make all ast and sem pointers const

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>

tools/src/cmd/remote-compile/main.cc

@@ -168,7 +168,7 @@ struct Message {
 
   explicit Message(Type ty) : type(ty) {}
 
-  Type const type;
+  const Type type;
 };
 
 struct ConnectionResponse : Message {  // Server -> Client