Fix 24208, 24212, 24213 - scope escape via pure + nested context

[pbackus]

The compiler sometimes allows scope pointers to be passed to non-scope parameters of pure function, with the exact rules documented in this section of the language spec:

https://dlang.org/spec/function.html#pure-scope-inference

Unfortunately, when checking for parameters with mutable indirections that an argument could escape through, the compiler previously:

• did not include variables accessed through the function's nested context, and
• did not assume conservatively that escape was possible if the function had a context that could vary at runtime.

As a result, it was possible to escape a scope pointer in @safe code.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	24208	normal	[DIP1000] Scope pointer can escape via non-scope parameter of pure nested function
✓	24212	normal	[DIP1000] Scope pointer can escape via non-scope parameter of pure virtual function
✓	24213	normal	[DIP1000] Scope pointer can escape via non-scope parameter of pure delegate

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#15756"

[pbackus]

Force push to re-run CI in case the problem with FreeBSD 13 is transient.

[thewilsonator]

This is a safety bug, did you want to target stable?

[pbackus]

There isn't another point release before 2.106, right? So it should be the same either way.

[ibuclaw]

There isn't another point release before 2.106, right? So it should be the same either way.

It's been a month since the last point release, so yes there might as well be another 2.105 release.