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Originally, MLIR only supported functions of the built-in FunctionType. On the conversion path to LLVM IR, we were creating MLIR functions that contained LLVM dialect operations and used LLVM IR types for everything expect top-level functions (e.g., a second-order function would have a FunctionType that consume or produces a wrapped LLVM function pointer type). With MLIR functions becoming operations, it is now possible to introduce non-built-in function operations. This will let us use conversion patterns for function conversion, simplify the MLIR-to-LLVM translation by removing the knowledge of the MLIR built-in function types, and provide stronger correctness verifications (e.g. LLVM functions only accept LLVM types). Furthermore, we can currently construct a situation where the same function is used with two different types: () -> () when its specified and called directly, and !llvm<"void ()"> when it's passed somewhere on called indirectly. Having a special function-op that is always of !llvm<"void ()"> type makes the function model and the llvm dialect type system more consistent. Introduce LLVMFuncOp to represent a function in the LLVM dialect. Unlike standard FuncOp, this function has an LLVMType wrapping an LLVM IR function type. Generalize the common behavior of function-defining operations (functions live in a symbol table of a module, contain a single region, are iterable as a list of blocks, and support argument attributes). This only defines the operation. Custom syntax, conversion and translation rules will be added in follow-ups. The operation name mentions LLVM explicitly to avoid confusion with standard FuncOp, especially in multiple files that use both `mlir` and `mlir::LLVM` namespaces. PiperOrigin-RevId: 259550940