Accept -x objc++-cpp-output as an alias for -x objective-c++-cpp-output

This is the ObjC++ version of r129201. It's for example needed to use
ccache with clang.

llvm-svn: 137560
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index 88c972c..bc6b08d 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -1322,6 +1322,7 @@
       .Case("objective-c-cpp-output", IK_PreprocessedObjC)
       .Case("objc-cpp-output", IK_PreprocessedObjC)
       .Case("objective-c++-cpp-output", IK_PreprocessedObjCXX)
+      .Case("objc++-cpp-output", IK_PreprocessedObjCXX)
       .Case("c-header", IK_C)
       .Case("objective-c-header", IK_ObjC)
       .Case("c++-header", IK_CXX)