Introduce LLVMFuncOp

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