Fix ThreadSafeRefCountedBase*::Release() (#3673)
C++ atomic's fetch_sub returns the previous value, where we want to
check the new value. This was causing massive memory leaks in SourceKit.
For ThreadSafeRefCountedBase, just switch to the one in LLVM that's
already correct. We should move the VPTR one to LLVM as well and then
we can get rid of this header.
rdar://problem/27358273
