Fix clang crash: when CodeGenAction is initialized without a context, use the member and not the parameter

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 263273

clang/lib/CodeGen/CodeGenAction.cpp

@@ -660,7 +660,7 @@ CodeGenAction::CodeGenAction(unsigned _Act, LLVMContext *_VMContext)
 #ifdef NDEBUG
   // FIXME: change this to be controlled by a cc1 flag that the driver passes,
   // on the model of --disable-free
-  _VMContext->setDiscardValueNames(true);
+  VMContext->setDiscardValueNames(true);
 #endif
 }