Fix function_cache garbage collection bug

[JoshuaBatty]

Description

Fixes a bug that would crash the language server when a function node was part of the AST.

#5967 added a function cache to the QueryEngine. Garbage collection needed to be called on this otherwise it returns references to types that have been cleared from the other engines during garbage collection.

In the future, any other nodes that we decide to cache need to have GC applied to them or we will end up crashing the language server on the first key-pressed event.

I've also removed the gc_frequency setting and now run GC on each keystroke. Without doing this, the spans stored in the TokenMap and what was returned from the compiler were falling out of sync. This ensures that everything is always up to date and the correct spans are used. closes #5260

Finally, I've added a launch.json that allows for attaching lldb to a live running forc-lsp process. This was a pretty cruical step for tracking down this bug so would be nice to have it easily accessible for future debugging sessions.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[codspeed-hq]

CodSpeed Performance Report

Merging #6555 will degrade performances by 18.37%

_{Comparing josh/fix_function_slab_gc (cddd31a) with master (e82c72b)}

Summary

❌ 1 regressions
✅ 21 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	master	josh/fix_function_slab_gc	Change
❌	document_symbol	4.3 ms	5.3 ms	-18.37%

[tritao]

Nice sleuthing on this

[sdankel]

Nice work!

[mchristopher]

Small thing to confirm, otherwise looks great

[JoshuaBatty]

Thanks, i'm going to merge this PR and then follow up with the suggestions for the launch.json file in a follow up PR.