go/types, types2: handle unconstrained type parameters correctly in a few places

When iterating over a type set via a range clause, am unconstrained
type set produces a single (nil, nil) result. This was not properly
accounted for in a few places:

- In the code for the append and copy built-in, handle the (nil, nil)
  case.
- Likewise, in NewSignatureType, panic with the correct (string) error
  in this case.

Check all remaining places where we iterate over type sets with range
for correctness.

Fixes #78163.

Change-Id: If0f33f43dad59b4b5ef4c310f80522c25c6e251f
Reviewed-on: https://go-review.googlesource.com/c/go/+/755941
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>

0 files changed, 0 insertions, 0 deletions