cmd/compile/internal/types2: better error position for instantiation failure

cmd/compile/internal/types2: better error position for instantiation failure

- Thread type argument expressions (rather than posLists) through various
type-checker functions so we can provide a better error position.

- Adjust signatures that expect a syntax.Pos to accept a poser instead
to avoid gratuituous conversions from expressions to positions.

- Rename targsx to xlist so we use xlist consistently for expression
lists.

First step in providing a better error message for the issue below.

For #49179.

Change-Id: I8fc685a2ee4f5640f4abd35568ba32bcb34e9e84
Reviewed-on: https://go-review.googlesource.com/c/go/+/366757
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>