[test] Add a couple of IUO test cases

Author: hamishknight

test/Constraints/dynamic_lookup.swift

@@ -311,6 +311,9 @@ let _: DynamicIUO = o[dyn_iuo]!
 let _: DynamicIUO = o[dyn_iuo]!!
 let _: DynamicIUO? = o[dyn_iuo]
 // FIXME: These should all produce lvalues that we can write through
+o.t = s // expected-error {{cannot assign to property: 'o' is immutable}}
+o.t! = s // expected-error {{cannot assign through '!': 'o' is immutable}}
+o.t!! = s // expected-error {{cannot assign through '!': 'o' is immutable}}
 o[dyn_iuo] = dyn_iuo // expected-error {{cannot assign through subscript: 'o' is immutable}}
 o[dyn_iuo]! = dyn_iuo // expected-error {{cannot assign through '!': 'o' is immutable}}
 o[dyn_iuo]!! = dyn_iuo // expected-error {{cannot assign through '!': 'o' is immutable}}

test/Constraints/iuo.swift

@@ -247,3 +247,7 @@ func rdar83352038() {
     return foo(cnode)
   }
 }
+
+// Make sure we reject an attempt at a function conversion.
+func returnsIUO() -> Int! { 0 }
+let _ = (returnsIUO as () -> Int)() // expected-error {{cannot convert value of type '() -> Int?' to type '() -> Int' in coercion}}