[clang-tools-extra] Re-add path normalization that was removed in 315561c86778 (PR #118718)

[Nicolas van Kempen via cfe-commits]

nicovank wrote:

Is there a reason to bring it back? Is it causing issues?
IIRC this was on purpose because from `os.path.abspath`:
> On most platforms, this is equivalent to calling the function [normpath()](https://docs.python.org/3/library/os.path.html#os.path.normpath) as follows: normpath(join(os.getcwd(), path)).

https://github.com/llvm/llvm-project/pull/118718