go/types: add Func.Signature method

go/types: add Func.Signature method

Unfortunately we can't enforce the repr invariant
that Func.typ != nil without thinking about the
object color invariants. For now, return a trivial
Signature if typ == nil, which should never happen
in bug-free client code.

Fixes golang/go#65772

Change-Id: I7f89c6d8fdc8dcd4b8880572e54bb0ed9b6136eb
Reviewed-on: https://go-review.googlesource.com/c/go/+/565375
Commit-Queue: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>