Clean up unique lock files on signal and always release the lock

Make sure to remove the unique lock file, which is what the .lock
symlink points to, if there is a signal while the lock is held. This
will release the lock, since the symlink will point to nothing (already
tested in unit tests). For good measure, also clean up the unique lock
file if there is an error or signal before the lock is acquired.

I will add a clang test.

rdar://problem/21512307

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240967 91177308-0d34-0410-b5e6-96231b3b80d8