Refactor tryCaptureVar using ExtractMethod.  No functionality change.

In chicago, Doug had requested that I go ahead and commit the refactor  as a separate change, if all the tests passed.  

Lets hope the buildbots stay quiet.

Thanks!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192087 91177308-0d34-0410-b5e6-96231b3b80d8