[RISCV][NFC] Rename variable in RISCVVEmitter.cpp

- Use `Overloaded` rather than `Mangled`.
- Use `Prototype` or `Desc` rather than `Seq`, it's not just a string sequence.
  - `Prototype` for those PrototypeDescriptor will used to evaluate as argument type.
  - `Desc` for those PrototypeDescriptor will used to evaluate as part of function name.

Reviewed By: khchen

Differential Revision: https://reviews.llvm.org/D125886