[CIR][Dialect] Add calling convention attribute to cir.call op

[seven-mile]

The first patch to fix #803 . This PR adds the calling convention attribute to CallOp directly, which is similar to LLVM, rather than adding the information to function type, which mimics Clang AST function type.

The syntax of it in CIR assembly is between the function type and extra attributes, as follows:

%1 = cir.call %fnptr(%a) : (!fnptr, !s32i) -> !s32i cc(spir_kernel) extra(#fn_attr)

The verification of direct calls is not included. It will be included in the next patch extending CIRGen & Lowering.

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For every builder method of Call Op, an optional parameter callingConv is inserted right before the parameter of extra attribute. However, apart from the parser / printer, this PR does not introduce any functional changes.

[sitio-couto]

@seven-mile quick question about this: what led you to add calling convention to the CallOp instead of the FuncOp? Or, in other words, choosing the more LLVM-like approach than the AST-like one?

[seven-mile]

FuncOp already got one, do you mean FunctionType? If so, the description of #803 provides more information about the decision.

[bcardosolopes]

This approach makes sense to me, I don't think adding this to the func type is the right approach.

Verification might be a bit tricky because we might start getting legit failures in code where people are relying on bad behavior (ODR violations). I'd say we should do verification anyways (your next PRs) and if/when it becomes a problem, we introduce a pass to do the "ODR hand-waive".

LGTM with minor nit.