[AST] Don't return decls from unsatisfied conformance
When a type conditionally conforms to a protocol, it used to provide
symbols from extension to that protocol.
e.g.:
protocol P {}
extension P {
func foo() {}
}
struct S<T> {}
extension S: P where T == Int {}
func test(val: S<String>) {
val.#^COMPLETE^#
}
This should not provide `foo()` method.
rdar://problem/36594731
