Fix crash in ObjC codegen introduced with 5ab6ee7

5ab6ee7 assumed that if `RValue::isScalar()` returns true then `RValue::getScalarVal` will return a valid value.  This is not the case when the return value is `void` and so void message returns would crash if they hit this path.  This is triggered only for cases where the nil-handling path needs to do something non-trivial (destroy arguments that should be consumed by the callee).

Reviewed By: triplef

Differential Revision: https://reviews.llvm.org/D123898

clang/lib/CodeGen/CGObjCGNU.cpp

@@ -2837,11 +2837,13 @@ CGObjCGNU::GenerateMessageSend(CodeGenFunction &CGF,
     // Enter the continuation block and emit a phi if required.
     CGF.EmitBlock(continueBB);
     if (msgRet.isScalar()) {
-      llvm::Value *v = msgRet.getScalarVal();
-      llvm::PHINode *phi = Builder.CreatePHI(v->getType(), 2);
-      phi->addIncoming(v, nonNilPathBB);
-      phi->addIncoming(CGM.EmitNullConstant(ResultType), nilPathBB);
-      msgRet = RValue::get(phi);
+      // If the return type is void, do nothing
+      if (llvm::Value *v = msgRet.getScalarVal()) {
+        llvm::PHINode *phi = Builder.CreatePHI(v->getType(), 2);
+        phi->addIncoming(v, nonNilPathBB);
+        phi->addIncoming(CGM.EmitNullConstant(ResultType), nilPathBB);
+        msgRet = RValue::get(phi);
+      }
     } else if (msgRet.isAggregate()) {
       // Aggregate zeroing is handled in nilCleanupBB when it's required.
     } else /* isComplex() */ {

clang/test/CodeGenObjC/gnustep2-nontrivial-destructor-argument.mm

@@ -0,0 +1,22 @@
+// RUN: %clang_cc1 -triple x86_64-unknow-windows-msvc -S -emit-llvm -fobjc-runtime=gnustep-2.0 -o - %s 
+
+// Regression test.  Ensure that C++ arguments with non-trivial destructors
+// don't crash the compiler.
+
+struct X
+{
+  int a;
+  ~X();
+};
+
+@protocol Y
+- (void)foo: (X)bar;
+@end
+
+
+void test(id<Y> obj)
+{
+  X a{12};
+  [obj foo: a];
+}
+