[Clang][AArch64] Fix some target guards and remove +sve from tests. (#80681)

The TargetGuard fields for 'svldr[_vnum]_za' and 'svstr[_vnum]_za' were
incorrectly set to `+sve` instead of `+sme`. This means that compiling
code that uses these intrinsics requires compiling for both `+sve` as
well as `+sme`.

This PR also fixes the target guards for the `svadd` and `svsub`
builtins that are enabled under `+sme2,+sme-i16i64` and
`+sme2,+sme-f64f64`, as it initially did the following:
```
  let TargetGuard = "+sme2" in {
    let TargetGuard = "+sme-i16i64" in {
      // Builtins defined here will be predicated only by
      // '+sme-i16i64', and not '+sme2,+sme-i16i64'.
    }
  }
```
This PR also removes `-target-feature +sve` from all the SME tests, to
ensure that the SME features are sufficient to build the tests.