C++: Move the identical-files.json spec to ql repo
C++: Move the identical-files.json spec to ql repo

This file is used by the `sync-identical-files.py` pull-request check in
our internal repo, which can hopefully soon start running on this repo
as well. This initial commit moves over all the file group definitions
whose files are entirely within this repository.

This change is not synchronized with the internal repo, so the file
groups will appear in both repositories until they sync up. That should
not cause any problems.

github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql