[clang] Make make_cxx_dr_status script runnable from anywhere

This script has hardcoded relative paths to `clang/test/CXX/drs`, `cwg_index.html`, and `cxx_dr_status.html`, which requires running it with `clang/www` CWD. This patch makes those paths relative to path of the script itself, so that it could be run from anywhere.

Reviewed By: #clang-language-wg, cor3ntin

Differential Revision: https://reviews.llvm.org/D148146