Mutates a WGSL-like string by replacing a randomly-selected identifier
with a different randomly-selected identifier.
Change-Id: Iecf45ad2800677cf3609b30d415520e5f2a05ba0
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/60561
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Alastair Donaldson <afdx@google.com>
Commit-Queue: Alastair Donaldson <afdx@google.com>