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.

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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).
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[redsun82]

Sounds good to me!

[geoffw0]

Couple of tiny points, otherwise LGTM.

[geoffw0]

Do we need to worry about files in subdirectories of the TryCompile-* directory? i.e. CMakeFiles/CMAkeScratch/TryCompile-foo/bar?

[jketema]

No. My understanding is that CMake creates each directory on-the-fly and only uses it for one test compilation.