[RISCV][clang] Optimize memory usage of intrinsic lookup table (#77487)

This patch optimize:
  1. Reduce string size of RVVIntrinsicDef.
  2. Reduce the type size of the index of intrinsics.

I use valgrind --tool=massif to analyze a simple program:
```
#include <riscv_vector.h>
vint32m1_t test(vint32m1_t v1, vint32m1_t v2, size_t vl) {
  return __riscv_vadd(v1, v2, vl);
}
```
and before optimization, the peak memory usage is 15.68MB,
after optimization, the peak memory usage is 13.69MB.