go/types, types2: better error message when using *interface instead of interface
- detect *interface case and report specific error
- replaced switch with sequence of if's for more clarity
- fixed isInterfacePtr: it applies to all interfaces, incl.
type parameters
- reviewed/fixed all uses of isInterfacePtr
- adjusted error messages to be consistently of the format
"type %s is pointer to interface, not interface"
Fixes #48312.
Change-Id: Ic3c8cfcf93ad57ecdb60f6a727cce9e1aa4afb5d
Reviewed-on: https://go-review.googlesource.com/c/go/+/376914
Trust: Robert Griesemer <gri@golang.org> Reviewed-by: Robert Findley <rfindley@google.com>