[PowerPC][Power10] Implement Vector Clear Left/Rightmost Bytes Builtins in LLVM/Clang

This patch implements builtins for the following prototypes:
```
vector signed char vec_clrl (vector signed char a, unsigned int n);
vector unsigned char vec_clrl (vector unsigned char a, unsigned int n);
vector signed char vec_clrr (vector signed char a, unsigned int n);
vector signed char vec_clrr (vector unsigned char a, unsigned int n);
```

Differential Revision: https://reviews.llvm.org/D81707