Thread safety analysis: Always warn when dropping locks on back edges

We allow branches to join where one holds a managed lock but the other
doesn't, but we can't do so for back edges: because there we can't drop
them from the lockset, as we have already analyzed the loop with the
larger lockset. So we can't allow dropping managed locks on back edges.

We move the managed() check from handleRemovalFromIntersection up to
intersectAndWarn, where we additionally check if we're on a back edge if
we're removing from the first lock set (the entry set of the next block)
but not if we're removing from the second lock set (the exit set of the
previous block). Now that the order of arguments matters, I had to swap
them in one invocation, which also causes some minor differences in the
tests.

Reviewed By: delesley

Differential Revision: https://reviews.llvm.org/D104261