Fixes #19012.

Fallback to return signatures without detailed types.
These error message will be of the form of issue:
* https://golang.org/issues/4215
* https://golang.org/issues/6750

So:
func f(x int, y uint) {
    return x > y
}

f(10, "a" < 3)

will give errors:
too many errors to return
too many arguments in call to f

instead of:

too many errors to return
  have (<T>)
  want ()
too many arguments in call to f
  have (number, <T>)
  want (number, number)

Change-Id: I680abc7cdd8444400e234caddf3ff49c2d69f53d
Reviewed-on: https://go-review.googlesource.com/36806Reviewed-by: Robert Griesemer <gri@golang.org>