Diagnose timeouts in the LockFileManager and delete the dead lock file

If the lock file manager times out, we should give an error rather than
silently trying to load the existing module.  And delete the
(presumably) dead lock file, since it will otherwise prevent progress in
future invokations. This is unsound since we have no way to prove that
the lock file we are deleting is the same one we timed out on, but since
the lock is only to avoid excessive rebuilding anyway it should be okay.
Depends on llvm r228603.

llvm-svn: 228604