[Clang][RISCV][NFC] Reorganize test case for rvv intrinsics

The file hierarchy is reorganized into:

```
├── rvv-intrinsics-autogenerated
│   ├── non-policy
│   │   ├── non-overloaded
│   │   └── overloaded
│   └── policy
│       ├── non-overloaded
│       └── overloaded
└── rvv-intrinsics-handcrafted
```

Separating auto-generated test cases and hand-craft ones. The
auto-generated ones are basic API tests generated from the intrinsic
generator [0].

This re-organization allows direct copy-and-paste from the produced
outputs of the generator in future API changes, which is discussed
and needs to be implemented towards a v1.0 RVV intrinsic.

[0] https://github.com/riscv-non-isa/rvv-intrinsic-doc/pull/181

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D141198