internal: pass db_lt to the inner_fn

[davidbarsky]

This is necessary to support passing interned structs to queries in the query group imitation macro: https://github.com/davidbarsky/db-ext-macro/.

(I'll upstream that macro to Salsa, promise! The code quality is bad, however...)

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[codspeed-hq]

CodSpeed Performance Report

Merging #617 will not alter performance

_{Comparing davidbarsky:david/pass-db-lt-to-inner-fn (c2ce00d) with master (32f8ee9)}

Summary

✅ 8 untouched benchmarks

[MichaReiser]

Could you add demonstrating its use case?

[davidbarsky]

Pushed a fix for the unused lifetime lint.

Could you add demonstrating its use case?

@MichaReiser Uh, probably. Without porting my macro over, it might be a little challenging, but I think I can add it as a desugared version.