cmd/compile, types2: better error message for invalid type assertion
This CL addresses the 2nd part of the issue below.
- For types2, now use the same error messages as the compiler in this case.
- Make the mechanism for reporting clarifying error messages handle the case
where we don't have additional position information.
- Provide context information (type assertion vs type switch).
Fixes #49005.
Change-Id: I4eeaf4f0c3f2f8735b63993778f58d713fef21ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/356512
Trust: Robert Griesemer <gri@golang.org> Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com> Reviewed-by: Robert Findley <rfindley@google.com>