lit: Provide source locations in cfg files with older Python versions

This commit prospectively brings the benefits of r198766 to older supported
Python versions (2.5+).

Tested with Python 2.6, 2.7, 3.1 and 3.3 (!)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199009 91177308-0d34-0410-b5e6-96231b3b80d8