cmd/compile/internal/types2: adjust printing of type parameter in error
For constraint type inference failures where the type parameter doesn't
match the constraint, print the type parameter rather than its type name
object. This provides more flexibility for improving the error message
down the road.
Change-Id: I188871d6f26a16cd96e59770966a1ec65607b128
Reviewed-on: https://go-review.googlesource.com/c/go/+/360514
Trust: Robert Griesemer <gri@golang.org> Reviewed-by: Robert Findley <rfindley@google.com>