CloneContext clones the AST, types, symbols and in the future semantic info.
3/4 of these are non-ast, so promote these up to the root.
Bug: tint:390
Change-Id: I49619796e6f81f9ab64f79413a12c87312cb1901
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/38361
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Ben Clayton