cmd/compile: tip regression: "incompatible type" error message should include more info

[rogpeppe]

commit a83a558

When a type isn't compatible with an interface type because it doesn't implement the required methods, the compiler used to mention at least one method that's messing. With the latest tip that's no longer the case - it just says "incompatible type" with no hint as to what's wrong.

For example, with this code:

package main

type I interface {
	Foo()
	Bar()
}

type S struct{}

func (S) Foo() {
}

func main() {
	var _ I = S{}
}

Go 1.17 prints the error:

./tst.go:14:6: cannot use S{} (type S) as type I in assignment:
	S does not implement I (missing Bar method)

but Go tip just prints:

./tst.go:14:12: incompatible type: cannot use S{} (value of type S) as I value

Historically I've found the additional information extremely helpful, so I think it should be reinstated.
Aside: I'd actually go further than that - I've often thought that it would be better if the error
showed all the missing methods, not just some arbitrary method, because that gives a better
idea of what's going on.

[cuonglm]

Seems duplicated of #48471

[rogpeppe]

@cuonglm Thanks! I agree that this is a dupe.