A find or return-default utility.
Rename `find_or_replace.h` to `map.h` which contains both these utilties that operate on maps.
Change-Id: Iaa76ea3f5c5a1210e413c131dd0556c126ee5d0a
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/70521
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>