Fix build: use -> with pointers and not .

Silly typo.

From: Mehdi Amini <mehdi.amini@apple.com>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@263267 91177308-0d34-0410-b5e6-96231b3b80d8

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
 }