Resolver: Track storage class usages of structures

This will be used to validate layout rules, as well as preventing
illegal types from being used in a uniform / storage buffer.

Also: Cleanup logic around VariableDeclStatement
This was spread across 3 places, entirely unnecessarily.

Bug: tint:643
Change-Id: I9d309c3a5dfb5676984f49ce51763a97bcac93bb
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/45125
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: James Price <jrprice@google.com>
Reviewed-by: David Neto <dneto@google.com>

src/ast/storage_class.h

@@ -34,6 +34,12 @@ enum class StorageClass {
   kFunction
 };
 
+/// @returns true if the StorageClass is host-sharable
+/// @see https://gpuweb.github.io/gpuweb/wgsl.html#host-shareable
+inline bool IsHostSharable(StorageClass sc) {
+  return sc == ast::StorageClass::kUniform || sc == ast::StorageClass::kStorage;
+}
+
 std::ostream& operator<<(std::ostream& out, StorageClass sc);
 
 }  // namespace ast