absl::FastTypeId<Type>() evaluates at compile-time to a unique id for the
passed-in type. These are meant to be good match for keys into maps or
straight up comparisons.
Previously this was an internal implemention detail used by a few libraries.
This is now a supported utility function.
PiperOrigin-RevId: 743194478
Change-Id: Ie01ed8a2850696ee661acb91726252adb4c20538