[Cherry-pick][clang] Correct behavior of LLVM_UNREACHABLE_OPTIMIZE=OFF for Release builds

[MasterAwesome]

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);
}

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);
}

References

• Unable to build LLVM with clang on master/stable 1.73 rust#116397
• llvm@bffbe9a
• Cannot build obj.clangAST when LLVM_UNREACHABLE_OPTIMIZATIONS are turned off llvm/llvm-project#68237 / [clang] Correct behavior of LLVM_UNREACHABLE_OPTIMIZE=OFF for Release builds llvm/llvm-project#68284

[MasterAwesome]

Does this need to be cherry-picked onto the branch rustc/17.0-2023-07-29 as well? This is planned to be backported to release/17.x in upstream LLVM.

[cuviper]

This is planned to be backported to release/17.x in upstream LLVM.

That backport should preferably land first, and then we'll just get it next time we merge to our branch.

[MasterAwesome]

This is planned to be backported to release/17.x in upstream LLVM.

That backport should preferably land first, and then we'll just get it next time we merge to our branch.

Sounds good, I'll close this PR and will close rust-lang/rust#116397 when that fix gets merged into stable.