Request message eliding doesn't respect @OutputOnly fields

[imjasonh]

A method that takes a path parameter called "foo" and a request body with a field called "foo" has the request message and path parameter elided, assuming that the value of "foo" specified in the request body will be the same one that should be specified in the path.

This ignores the case where the "foo" field in the request body is marked as @OutputOnly, where it's not valid to pass the value in the request body.

--unelidable_request_methods can specify that the message should not be elided, but ideally the eliding logic would skip eliding messages when @OutputOnly is involved.

I believe the necessary change is somewhere in _NeedRequestType here.