go/types: split out function instantiation from index expr

This is a port of CL 308371 to go/types. The only meaningful change from
that CL is to use explicit return values in Checker.indexExpr, which I
felt was more readable. I made the same change in types2 to keep them in
sync

Change-Id: I3380c03fe49d3bf4167cadad305abe942785af19
Reviewed-on: https://go-review.googlesource.com/c/go/+/314432


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>