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

clang/docs/ReleaseNotes.rst

@@ -719,6 +719,8 @@ Bug Fixes in This Version
   virtual member functions even if the target required a greater function
   alignment and/or did not have function pointers which point to function entry
   points (i.e., uses function descriptor objects instead).
+- Fixes a ``clang-17`` regression where ``LLVM_UNREACHABLE_OPTIMIZE=OFF``
+  cannot be used with ``Release`` mode builds. (`#68237 <https://github.com/llvm/llvm-project/issues/68237>`_).
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/utils/TableGen/ClangAttrEmitter.cpp

@@ -2639,23 +2639,28 @@ static void emitAttributes(RecordKeeper &Records, raw_ostream &OS,
         OS << ", ";
         emitFormInitializer(OS, Spellings[0], "0");
       } else {
-        OS << ", (\n";
+        OS << ", [&]() {\n";
+        OS << "    switch (S) {\n";
         std::set<std::string> Uniques;
         unsigned Idx = 0;
         for (auto I = Spellings.begin(), E = Spellings.end(); I != E;
              ++I, ++Idx) {
           const FlattenedSpelling &S = *I;
           const auto &Name = SemanticToSyntacticMap[Idx];
           if (Uniques.insert(Name).second) {
-            OS << "    S == " << Name << " ? AttributeCommonInfo::Form";
+            OS << "    case " << Name << ":\n";
+            OS << "      return AttributeCommonInfo::Form";
             emitFormInitializer(OS, S, Name);
-            OS << " :\n";
+            OS << ";\n";
           }
         }
-        OS << "    (llvm_unreachable(\"Unknown attribute spelling!\"), "
-           << " AttributeCommonInfo::Form";
+        OS << "    default:\n";
+        OS << "      llvm_unreachable(\"Unknown attribute spelling!\");\n"
+           << "      return AttributeCommonInfo::Form";
         emitFormInitializer(OS, Spellings[0], "0");
-        OS << "))";
+        OS << ";\n"
+           << "    }\n"
+           << "  }()";
       }
 
       OS << ");\n";