Add a bit of a hack to deal with a failing testcase on darwin10 bots.

Add a bit of a hack to deal with a failing testcase on darwin10 bots.

We currently emit an error message when you try to use thread local
storage on targets that don't support it and testing C++11 thread
locals will trip this. We don't want to xfail the test for all darwin
hosts so add a quick hack to check for darwin10 and disable the
test based on that. Only checking darwin10 because anything earlier
is really old and I don't have a list of what other hosts don't
support tls handy.

Alternate suggestions welcome!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179671 91177308-0d34-0410-b5e6-96231b3b80d8