C++: Add taint through fopen

[MathiasVP]

Pull Request checklist

Ideally this should've been done through MaD, but considering that we had the Fopen class already I couldn't really be bothered to convert it to MaD 😂

I'll run DCA once that's unbroken.

All query authors

• A change note is added if necessary. See the documentation in this repository.
• All new queries have appropriate .qhelp. See the documentation in this repository.
• QL tests are added if necessary. See Testing custom queries in the GitHub documentation.
• New and changed queries have correct query metadata. See the documentation in this repository.

Internal query authors only

• Autofixes generated based on these changes are valid, only needed if this PR makes significant changes to .ql, .qll, or .qhelp files. See the documentation (internal access required).
• Changes are validated at scale (internal access required).
• Adding a new query? Consider also adding the query to autofix.

[geoffw0]

LGTM.

[MathiasVP]

One new result (I had expected more really 😭), but a least it looks like a TP. Merging!