cmd/compile/internal/types2: deduplicate signatures with the context
This CL is a mostly clean port of CL 362801 from go/types to types2.
It deviates from go/types in some of the testing code because types2
already had made some of the changes.
It also re-introduces some empty lines that got lost in earlier CLs.
Change-Id: I0bebd68f0880fac61631a5d0c323a9f8ce853ac6
Reviewed-on: https://go-review.googlesource.com/c/go/+/364335
Trust: Robert Griesemer <gri@golang.org> Reviewed-by: Robert Findley <rfindley@google.com>