scip-ctags: add kinds support for C

[mrnugget]

This is part of https://github.com/sourcegraph/sourcegraph/issues/58024 and adds scip-ctags support for C.

Test plan

• Unit tests

[varungandhi-src]

There is a separate StaticVariable Kind here: https://sourcegraph.com/github.com/sourcegraph/scip@main/-/blob/bindings/typescript/scip.ts?L1353:13

[varungandhi-src]

I've mentioned some missing test cases inline. Some more test cases to add:

• K&R style declarations. example
• Union declarations
• Make sure that we're not emitting symbols for type declarations in nested contexts (such as inside another type definition, inside a function body etc, inside a parameter list) (as those aren't accessible from a global scope). E.g. struct z { struct w { int x; int y; } b; } -- we shouldn't emit a global descriptor for w here.

[mrnugget]

Thanks @varungandhi-src! union popped into my head earlier, but not the other things. I'm on it.

[mrnugget]

Another thing: pointers to pointers!

struct connection **my_ptr_ptr;

[mrnugget]

Okay, in the name of simplicity I simplified a lot of things:

• Added "support" for most of the constructs mentioned above
• Dropped "Constants" as a separate kind, because it made the queries essentially unmaintainable (due to recursive nature of declarations and the modifier being top-level).
• Queries are much simpler now, at the cost of fidelty: we don't tag StaticVariable, we don't tag Constant -- but we correctly tag a const int ****ptrptrptrptr;
• Dropped the whole thing of fields in anonymous unions shadowing global variables, because that would require modifying the Rust code.
• Added tests for scip and tags.

I think that's in the spirit of what we're after here, @jtibshirani, right? Let me know if we aim for more correctness, then I'll take another stab.

Otherwise, I think we can always follow-up and tweak some things.

[jtibshirani]

@mrnugget the simplifications look great and seem like the right trade-off. Our intended use cases for symbol kinds don't require a high level of precision (it's just ranking and showing the kinds for symbol search). I'll review again shortly.

[mrnugget]

Update: talked with @tjdevries a bit and he also confirmed that getting const/static working reliably and fixing the anonymous union thing would require more than just a couple of hours of query writing -- would need to change the Rust code. Happy to do that, of course, but wanted to check, @jtibshirani, whether that's even wanted or not.

[jtibshirani]

Looks good to me! I see all the important kinds to detect.