Add location specifier to MLIR Functions, and:

Add location specifier to MLIR Functions, and:
 - Compress the identifier/kind of a Function into a single word.
 - Eliminate otherFailure from verifier now that we always have a location
 - Eliminate the error string from the verifier now that we always have
   locations.
 - Simplify the parser's handling of fn forward references, using the location
   tracked by the function.

PiperOrigin-RevId: 211985101