[analyzer] Don't crash if we cache out after making a temporary region.

A C++ overloaded operator may be implemented as an instance method, and
that instance method may be called on an rvalue object, which has no
associated region. The analyzer handles this by creating a temporary region
just for the evaluation of this call; however, it is possible that /by
creating the region/, the analyzer ends up in a previously-explored state.
In this case we don't need to continue along this path.

This doesn't actually show any behavioral change now, but it starts being
used with the next commit and prevents an assertion failure there.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179766 91177308-0d34-0410-b5e6-96231b3b80d8