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

**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.