[OMPT] Fix assertion that arises when waiting for proxy tasks on runtime shutdown

This only triggered when built in debug mode with OMPT enabled:
__kmp_wait_template expected the state of the current thread to be either
ompt_state_idle or ompt_state_wait_barrier{,_implicit,_explicit}.

Patch by Jonas Hahnfeld

Differential Revision: http://reviews.llvm.org/D12754

llvm-svn: 247339