Infer the return type by finding the common type across all cases.
Types that derive from CastableBase will automatically infer to
the common base class.
Change-Id: I2112ca1abae34e55396685e9ebf2da12f8a6e3fc
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/80320
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Auto-Submit: Ben Clayton <bclayton@google.com>