[clang-tidy] Fix add_new_check.py to generate correct list.rst autofix column from relative path

Previously, the code in add_new_check.py that looks for fixit keywords in check source files when generating list.rst assumed that the script would only be called from its own path. That means it doesn't find any source files for the checks it's attempting to scan for, and it defaults to writing out nothing in the "Offers fixes" column for all checks. Other parts of add_new_check.py work from other paths, just not this part.

After this fix, add_new_check.py's "offers fixes" column generation for list.rst will be consistent regardless of what path it's called from by using the caller path that's deduced elsewhere already from sys.argv[0].

Reviewed By: kbobyrev

Differential Revision: https://reviews.llvm.org/D110600