go/types: match types2 error for invalid map key

go/types: match types2 error for invalid map key

Use "invalid" rather than "incomparable" in error message for map key
types that are not comparable. This is the original compiler error
message and many tests check for this specific message. The type
checker does provide an additional explanation if the reason for
the error is not obvious (e.g. for type parameters).

For #54511.

Change-Id: Idb76c48b4dfbfd66a7deac728a552e07f14e06d7
Reviewed-on: https://go-review.googlesource.com/c/go/+/424905
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>