[Issue 10979] Add trait for function callgraph extraction, to allow "builtin attribute"-style UDA semantic checks

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=10979

--- Comment #7 from Adam D. Ruppe <destructionator at gmail.com> ---
Type functions aren't a substitute for this at all. Even if you had them,
they'd still need a way to introspect the function's calls which is what this
enhancement is requesting.

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