go/types: return an error from Instantiate

This is a port of CL 342152 to go/types. Additionally, a panic was
removed from interface substitution, which is a fix from CL 333155 that
was previously missed.

A check for a nil Checker was also removed from types2.instantiate,
since check must not be nil in that method.

Change-Id: I4ea6bdccbd50ea2008ee6d870f702bee5cdd5a8e
Reviewed-on: https://go-review.googlesource.com/c/go/+/342671


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>