Attach calling convention to `llvm.call` operations when lowering

[victor-eds]

Is your feature request related to a problem? Please describe

llvm.call now supports calling convention attributes. SYCL-MLIR was circumventing the lack of such attributes by taking the callee function calling convention. However, this would not work, e.g., for indirect calls.

Describe the solution you would like

Now that llvm.call supports calling convention attributes, attach them during conversion instead of taking the function calling convention when translating to LLVM IR.