Factor a bit of code to MCStreamer::EmitLabel. Keep track of the last

Factor a bit of code to MCStreamer::EmitLabel. Keep track of the last
non private symbol. This will be use for handling

foo:
  .cfi_startproc
  ...

On OS X where we have to create a foo.eh symbol.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@130305 91177308-0d34-0410-b5e6-96231b3b80d8