[Clang][AArch64]Add QCVTN builtin to SVE2.1 (#75454)

``` c
   // All the intrinsics below are [SVE2.1 or SME2]
   // Variants are also available for _u16[_s32]_x2 and _u16[_u32]_x2
   svint16_t svqcvtn_s16[_s32_x2](svint32x2_t zn);
   ```

According to PR#257[1]

[1]https://github.com/ARM-software/acle/pull/257