go/types: better error position for instantiation failure

go/types: better error position for instantiation failure

This is a port of CL 366757 from types2 to go/types,
adjusted for the different handling of index expressions
in go/types.

For #49179.

Change-Id: Ic859eb09683134d055e28c8e0cb1f3814a87dc5c
Reviewed-on: https://go-review.googlesource.com/c/go/+/367198
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>