Test stubbing a method of an anonymous interface.

-- go.mod --
module example.com

go 1.18

-- a/a.go --
package a

var _ interface{ f(int) } = C(0) //@quickfix(re"C", re"missing method", out)

type C int

-- @out/a/a.go --
@@ -7 +7,4 @@
+// f implements an anonymous interface.
+func (c C) f(int) {
+	panic("unimplemented")
+}
