[MLIR] Add support for calling conventions to LLVM::CallOp and LLVM::InvokeOp (#71319)

Despite the fact that the LLVM dialect’s `FuncOp` already supports
calling conventions, there was yet no support for them in the ops that
actually perform function calls, which led to incorrect LLVM IR being
generated if one actually tried setting a `FuncOp`’s calling convention
to anything other than `ccc`.

This commit adds support for calling conventions to `LLVM::CallOp` and
`LLVM::InvokeOp` and makes sure that calling conventions are parsed,
printed, and lowered appropriately.