Always create a temporary symbol to use with the cfi frame.

This is a small simplification and a small step in fixing pr18743 since
private functions on MachO should be using a 'l' prefix.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200994 91177308-0d34-0410-b5e6-96231b3b80d8