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

The problem on Windows was that the \b in "..\bin" was interpreted
as an escape sequence. Use r"" strings to prevent that.

This reverts commit ab11b9ee,
with raw strings in the lit.site.cfg.py.in files.

Differential Revision: https://reviews.llvm.org/D77184