[stable] Move fix for issue 22974 from glue layer to frontend

[kinke]

See #13950.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✗	22974	normal	importC: D name mangling applied to extern variable inside function

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#14045"

[kinke]

Oh wait, I missed that the || vd.mangleOverride was added in the meantime.

[maxhaton]

Oh wait, I missed that the || vd.mangleOverride was added in the meantime.

ok. Does that mean this isn't sufficiently covered by tests? If so could you add one (that would've caught this omission, if I follow correctly)

[kinke]

Looks like the test added in #14008 should cover it just fine. So I guess the if (d.mangleOverride) return null; is superfluous at the moment (for VarDecls).

Edit: Yep, mangle overrides are handled early in dmangle.d's Mangler, so I've changed it to an assertion.

[thewilsonator]

This good to go? (Also cc @ibuclaw)

[ibuclaw]

Yeah, seems reasonable to me.