Fix "control reaches end of non-void function" warning. NFCI.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@352192 91177308-0d34-0410-b5e6-96231b3b80d8

lib/AST/ExprObjC.cpp

@@ -315,6 +315,7 @@ QualType ObjCMessageExpr::getCallReturnType(ASTContext &Ctx) const {
   case VK_RValue:
     return QT;
   }
+  llvm_unreachable("Unsupported ExprValueKind");
 }
 
 SourceRange ObjCMessageExpr::getReceiverRange() const {