go/types: add untyped test cases for AssignableTo API
The AssignableTo API is specifically for non-constant values, but is
currently called by gopls for constant completions. Add a test to ensure
that we handle this edge case correctly.
Change-Id: I83115cbca2443a783df1c3090b5741260dffb78e
Reviewed-on: https://go-review.googlesource.com/c/go/+/250258
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Robert Griesemer <gri@golang.org>