Revert "[lit] Keep original cfg file case around."

This reverts commit bc3f54de.

The patch breaks in the following two scenarios:

1. When manually passing an absolute path to llvm-lit with a lower-case
   drive letter: `python bin\llvm-lit.py -sv c:\llvm-project\clang\test\PCH`

2. When the PWD has a lower-case drive letter, like after running
   `cd c:\` with a lower-case "c:" (cmd's default is upper-case, but
   it takes case-ness from what's passed to `cd` apparently).