Fix a subtle bug with cleanups: when activating

Fix a subtle bug with cleanups:  when activating
a previously-inactive cleanup, not only do we need a
flag variable, but we should also force the cleanup to
query the flag variable.  However, we only need to do
this when we're activating in a context that's
conditionally executed;  otherwise, we may safely
assume that the cleanup is dominated by the activation
point.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@144271 91177308-0d34-0410-b5e6-96231b3b80d8