Fix bug discovered with address sanitizer.

Now, this works again with an empty stack.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@178779 91177308-0d34-0410-b5e6-96231b3b80d8
Remove the unused MemoryBuffers, no functionality change.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@178780 91177308-0d34-0410-b5e6-96231b3b80d8