[dev.go2go] go/types: add test for crash 15 from issue 39634

This test case doesn't crash anymore.

Updates #39634.

Change-Id: I29b13762265ef1c16c0298d141fdb6cf2c01f4ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/241123
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/fixedbugs/issue39634.go2

@@ -27,6 +27,11 @@ type foo8(type A) interface { type A }
 func bar8(type A foo8(A))(a A) {}
 func main8() {}
 
+// crash 15
+func y15() { var a /* ERROR declared but not used */ interface{ p() } = G15(string){} }
+type G15(type X) s /* ERROR undeclared name */
+func (G15 /* ERROR generic type .* without instantiation */ ) p()
+
 // crash 16
 type Foo16(type T) r16 /* ERROR not a type */
 func r16(type T)() Foo16((Foo16(T)))