Added function in node_id_map to check a given id is valid and fresh.
Currently, the structure of FindMutators declares node_id_map as const, which causes issues when we want to call `GetFreshId` from the argument that is passed by reference. A simple work around is to pass a non-const node_id_map as argument directly. That way `GetFreshId` function in node_id_map can continue to be non-const and conveniently update next fresh id whenever a fresh id has been taken.
Change-Id: Ia7e1d247cf92dfefd2ef7e7c1b4bf32363d9ce3f
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/61100
Reviewed-by: Paul Thomson <paulthomson@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Paul Thomson <paulthomson@google.com>