C++: Silence alerts coming from CMake test compilation files

[jketema]

This silences alert wobble, as the suffix of CMake TryCompile- directories is random and changes each time a project is built.

There's probably other queries where this makes sense, but for now I've limited myself to queries for which I concretely observed TryCompile- files showing up.

DCA nightly shows that the neovim alerts on TryCompile- files disappears, and similarly for DCA autobuild posix and .NET diagnostics.

Pull Request checklist

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.