Introduce support for variadic function signatures for the LLVM dialect

LLVM function type has first-class support for variadic functions.  In the
current lowering pipeline, it is emulated using an attribute on functions of
standard function type.  In LLVMFuncOp that has LLVM function type, this can be
modeled directly.  Introduce parsing support for variadic arguments to the
function and use it to support variadic function declarations in LLVMFuncOp.
Function definitions are currently not supported as that would require modeling
va_start/va_end LLVM intrinsics in the dialect and we don't yet have a
consistent story for LLVM intrinsics.

PiperOrigin-RevId: 262372651