fix #21504 - __traits(compiles) affects inference of @nogc in functions

[ibuclaw]

Adds test for isRootTraitsCompilesScope within setGC, so that a compile-time evaluation of a GC action does not alter the inferred attributes of a function and its function type.

Closes: #21504

[dlang-bot]

Thanks for your pull request, @ibuclaw!

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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 "stable + dmd#21505"

[thewilsonator]

the issue the fixes is a blocker for a PR that is to stable, are you intending to cherry-pick this, or do you want to target stable here?

[thewilsonator]

Thanks!