go/ssa: canonicalize receiver types to avoid creating duplicate thunk functions

+ Tests:
  Extend function name uniqueness check to exported wrappers.
  Check that expected synthetic functions are created, reachable, and not duplicated.

Change-Id: I0e87ebb2712e33e1f49da3fa9a9dde0085bf3850
Reviewed-on: https://go-review.googlesource.com/2013
Reviewed-by: Robert Griesemer <gri@golang.org>