[clang] [libclang/python] Add LIBCLANG_LIBRARY_PATH and LIBCLANG_LIBRARY_FILE (PR #170201)

[Thomas Applencourt via cfe-commits]

TApplencourt wrote:

Ok, so:
- Added new tests: clang/bindings/python/tests/cindex/test_environment_variable.py
- Removed the old environments notions (update the README and the CINDEX error message)
- Updated the error message

Sorry I failed my `git amend` so the commit history is ugly, will make the review harder. 

https://github.com/llvm/llvm-project/pull/170201