[clang] Correct behavior of `LLVM_UNREACHABLE_OPTIMIZE=OFF` for `Release` builds (#68284)

# Codegen
### Before
```c++
AArch64SVEPcsAttr *AArch64SVEPcsAttr::CreateImplicit(ASTContext &Ctx, SourceRange Range, Spelling S) {
  AttributeCommonInfo I(Range, NoSemaHandlerAttribute, (
    S == GNU_aarch64_sve_pcs ? AttributeCommonInfo::Form{AttributeCommonInfo::AS_GNU, GNU_aarch64_sve_pcs, false /*IsAlignas*/, false /*IsRegularKeywordAttribute*/} :
    S == CXX11_clang_aarch64_sve_pcs ? AttributeCommonInfo::Form{AttributeCommonInfo::AS_CXX11, CXX11_clang_aarch64_sve_pcs, false /*IsAlignas*/, false /*IsRegularKeywordAttribute*/} :
    S == C23_clang_aarch64_sve_pcs ? AttributeCommonInfo::Form{AttributeCommonInfo::AS_C23, C23_clang_aarch64_sve_pcs, false /*IsAlignas*/, false /*IsRegularKeywordAttribute*/} :
    (llvm_unreachable("Unknown attribute spelling!"),  AttributeCommonInfo::Form{AttributeCommonInfo::AS_GNU, 0, false /*IsAlignas*/, false /*IsRegularKeywordAttribute*/})));
  return CreateImplicit(Ctx, I);
}
```

### After
```c++
AArch64SVEPcsAttr *AArch64SVEPcsAttr::CreateImplicit(ASTContext &Ctx, SourceRange Range, Spelling S) {
  AttributeCommonInfo I(Range, NoSemaHandlerAttribute, [&]() {
    switch (S) {
    case GNU_aarch64_sve_pcs:
      return AttributeCommonInfo::Form{AttributeCommonInfo::AS_GNU, GNU_aarch64_sve_pcs, false /*IsAlignas*/, false /*IsRegularKeywordAttribute*/};
    case CXX11_clang_aarch64_sve_pcs:
      return AttributeCommonInfo::Form{AttributeCommonInfo::AS_CXX11, CXX11_clang_aarch64_sve_pcs, false /*IsAlignas*/, false /*IsRegularKeywordAttribute*/};
    case C23_clang_aarch64_sve_pcs:
      return AttributeCommonInfo::Form{AttributeCommonInfo::AS_C23, C23_clang_aarch64_sve_pcs, false /*IsAlignas*/, false /*IsRegularKeywordAttribute*/};
    default:
      llvm_unreachable("Unknown attribute spelling!");
      return AttributeCommonInfo::Form{AttributeCommonInfo::AS_GNU, 0, false /*IsAlignas*/, false /*IsRegularKeywordAttribute*/};
    }
  }());
  return CreateImplicit(Ctx, I);
}
```

Fixes https://github.com/llvm/llvm-project/issues/68237