[ObjectiveC] -rewrite-objc was treating inputs as preprocessed files (#137623)

`-rewrite-objc` passes `-x objective-c++-cpp-output` as input type to the preprocessor job. This is not correct since we would be preprocessing a preprocessed file. The correct input type is `objective-c++`.

Author: jmmartinez

clang/lib/Driver/ToolChains/Clang.cpp

@@ -521,7 +521,7 @@ static void addDashXForInput(const ArgList &Args, const InputInfo &Input,
 
  CmdArgs.push_back("-x");
  if (Args.hasArg(options::OPT_rewrite_objc))
- CmdArgs.push_back(types::getTypeName(types::TY_PP_ObjCXX));
+ CmdArgs.push_back(types::getTypeName(types::TY_ObjCXX));
  else {
  // Map the driver type to the frontend type. This is mostly an identity
  // mapping, except that the distinction between module interface units

clang/test/Driver/rewrite-objc-preproc.m

@@ -0,0 +1,5 @@
+// RUN: %clang --target=x86_64-apple-macosx10.7.0 -rewrite-objc %s -o - -### 2>&1 | \
+// RUN: FileCheck %s
+//
+// Check that we're running a preprocessing stage passing a not-preprocessed objective-c++ file as input
+// CHECK: "-E"{{.*}}"-x" "objective-c++"