Add keypath write test for -swift-version 5.

Author: rudkx

test/Constraints/keypath_swift_5.swift

@@ -5,9 +5,17 @@ struct S {
 
   init() {
     let _: WritableKeyPath<S, Int> = \.i // expected-error {{type of expression is ambiguous without more context}}
+
+    S()[keyPath: \.i] = 1
+    // expected-error@-1 {{cannot assign to immutable expression}}
   }
 }
 
 func test() {
   let _: WritableKeyPath<C, Int> = \.i // expected-error {{type of expression is ambiguous without more context}}
+
+  C()[keyPath: \.i] = 1
+  // expected-error@-1 {{cannot assign to immutable expression}}
+
+  let _ = C()[keyPath: \.i] // no warning for a read
 }