Refactor argument attribute specification in intrinsic definition. NFC.

- Argument attribute needs specifiying through `ArgIndex<n>`
  (corresponding to `FirstArgIndex`) to distinguish explicitly from the
  index number from the overloaded type list.
- In addition, `RetIndex` (corresponding to `ReturnIndex`) and
  `FuncIndex` (corresponding to `FunctionIndex`) are introduced for us
  to associate attributes on the return value and potentially function
  itself.

Differential Revision: https://reviews.llvm.org/D80422