Make `cir.call` aware of calling conventions

[seven-mile]

LLVM has both callee (Function) and callsite (CallInst) attributed by calling conv. Currently CIR always set default calling conv (CConv::C) when lowering to LLVM call / invoke ops, which produces wrong LLVM IR for callsites.

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Here are some considerations from my side, FYI.

First of all, I'm not sure whether ClangIR should allow mismatch of call convs between callsite and callee: LLVM allows it, but I think Clang AST does not. In Clang AST, the calling conv is stored in type system (clang::FunctionType::ExtInfo). Thus, I lean towards the latter (they must match).

Then, we have two typical approaches here, again ; )

• Extend !cir.func<> type with calling convention attribute. Infer the call conv for callsite when lowering.
  • This provides a straightforward equivalent to Clang AST, and defers the conversion until lowering.
• Extend the cir.call operations directly and keep close with OG skeleton.
  • By utilizing the information from Clang AST, we can still easily determine the calling convention of the callsite in corner cases like invoking a function pointer.

I think both are workable for just correctness. But the first one preserve more information for analysis and cost more (more allocation size for every function type). FWIW: The so-called "more information" is the calling conventions of intermediate results of functions, which is not common. Most of the time, we are just doing bitcast of function pointers. I don't think such info involves vital optimizations.

[seven-mile]

Update: Note that LLVM also does NOT allow mismatch between call convs of call and func. It's undefined behavior and would be optimized into unreachable for some passes.