Rework the FirstIndexOffset transform

So that it transforms more on clone than in-place.

Bug: dawn:548
Bug: tint:390
Change-Id: I0127bc02c4e0e88c924042c491d274363422cc52
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/35420
Commit-Queue: dan sinclair <dsinclair@chromium.org>
Reviewed-by: dan sinclair <dsinclair@chromium.org>

src/ast/module.h

@@ -56,6 +56,10 @@ class Module {
   Module Clone();
 
   /// Clone this module into `ctx->mod` using the provided CloneContext
+  /// The module will be cloned in this order:
+  /// * Constructed types
+  /// * Global variables
+  /// * Functions
   /// @param ctx the clone context
   void Clone(CloneContext* ctx);