cmd/compile/internal/types2: catch unexpected expression lists

This is a modified port of the https://golang.org/cl/313909
change for go/types.

- add catch-all cases for unexpected expression lists
- add Checker.singleIndex function to check single indices
- better syntax error handling in parser for invalid type
  instantiations that are missing a type argument

Change-Id: I6f0f396d637ad66b79f803d886fdc20ee55a98b3
Reviewed-on: https://go-review.googlesource.com/c/go/+/314409


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>