Fix a regression I introduced back in r178147.

We don't want cast and dyn_cast to work on temporaries. They don't extend
lifetime like a direct bind to a reference would, so they can introduce
hard to find bugs.

I added tests to make sure we don't regress this. Thanks to Eli Friedman for
noticing this and for his suggestions on how to test it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186559 91177308-0d34-0410-b5e6-96231b3b80d8