Revert "Make it possible for lit.site.cfg to contain relative paths, and use it for llvm and clang"

This reverts commit fb80b6b2 and
follow-up 631ee8b2.

Seems to not work on Windows:
http://lab.llvm.org:8011/builders/llvm-clang-lld-x86_64-scei-ps4-windows10pro-fast/builds/31684
http://lab.llvm.org:8011/builders/llvm-clang-win-x-aarch64/builds/6512

Let's revert while I investigate.