[clang][CodeGen] fix UB in aarch64 bfloat16 scalar conversion (#89062)

do not bitcast 16bit `bfloat16` to 32bit `int32_t` directly
bitcast to `int16_t`, and then upcast to `int32_t`

Fix ASAN runtime error when calling vcvtah_f32_bf16
`==21842==ERROR: AddressSanitizer: stack-buffer-overflow on address
0x007fda1dd063 at pc 0x005c0361c234 bp 0x007fda1dd030 sp 0x007fda1dd028
`

without patch
```c
__ai __attribute__((target("bf16"))) float32_t vcvtah_f32_bf16(bfloat16_t __p0) {
  float32_t __ret;
bfloat16_t __reint = __p0;
int32_t __reint1 = *(int32_t *) &__reint << 16;
  __ret = *(float32_t *) &__reint1;
  return __ret;
}
```

with this patch
```c
__ai __attribute__((target("bf16"))) float32_t vcvtah_f32_bf16(bfloat16_t __p0) {
  float32_t __ret;
bfloat16_t __reint = __p0;
int32_t __reint1 = (int32_t)(*(int16_t *) &__reint) << 16;
  __ret = *(float32_t *) &__reint1;
  return __ret;
}
```

fix issue https://github.com/llvm/llvm-project/issues/61983