The main change is to eliminate the use of "Argument" terminology when
dealing with overloaded types since overloaded types can be either
argument or return values, and some additional renaming for clarity.
1. Rename `Tys` argument to various intrinsic APIs to `OverloadTys` to
better reflect its meaning.
2. Rename `IITDescriptorKind::Argument` to
`IITDescriptorKind::Overloaded` to better convey that it's an overloaded
type. Removed "Argument" suffix for other kinds for dependent types.
3. Rename `ArgKind` to `AnyKind`, `getArgumentNumber` to
`getOverloadIndex`, `getArgumentKind` to `getOverloadKind`,
`getRefArgNumber` to `getRefOverloadIndex`, and `IIT_ARG` to `IIT_ANY`.
4. Rename `IIT_ANYPTR` (used to represent a pointer qualified with
address space) to `IIT_PTR_AS` to clearly distinguish it from
`llvm_anyptr_ty`
5. Change the packing of [ref overload index & overload index] for
`VecOfAnyPtrsToElt` to pack the overload index into the lower bits, so
we can use the `getOverloadIndex` function to get the overload index.
99786f20ee