[CS] Fix logic determining parens for non-optional weak diagnostic

We were previously checking the optional type,
which would never say that parens were needed.
Unwrap the type before checking, ensuring we
correctly handle e.g protocol compositions.

Author: hamishknight

lib/Sema/CSDiagnostics.cpp

@@ -9124,7 +9124,7 @@ bool InvalidWeakAttributeUse::diagnoseAsError() {
                        ReferenceOwnership::Weak, varType);
 
   auto typeRange = var->getTypeSourceRangeForDiagnostics();
-  if (varType->hasSimpleTypeRepr()) {
+  if (varType->lookThroughSingleOptionalType()->hasSimpleTypeRepr()) {
     diagnostic.fixItInsertAfter(typeRange.End, "?");
   } else {
     diagnostic.fixItInsert(typeRange.Start, "(")

test/attr/attributes.swift

@@ -191,6 +191,12 @@ weak var weak16 : Class!
 
 @weak var weak17 : Class? // expected-error {{'weak' is a declaration modifier, not an attribute}} {{1-2=}}
 
+class SomeClass {}
+protocol SomeProtocol {}
+_ = {
+  // Make sure the fix-it here includes the parens
+  weak var x: SomeClass & SomeProtocol // expected-error {{'weak' variable should have optional type '(any SomeClass & SomeProtocol)?'}} {{15-15=(}} {{39-39=)?}}
+}
 
 @_exported var exportVar: Int // expected-error {{@_exported may only be used on 'import' declarations}}{{1-12=}}
 @_exported func exportFunc() {} // expected-error {{@_exported may only be used on 'import' declarations}}{{1-12=}}