[libc][WrapperGen] Replace the C _Noreturn annotation with C++ [[noreturn]].

[michaelrj]

LGTM

[sivachandra]

Fix typo.

libc/utils/tools/WrapperGen/Main.cpp

static bool WrapperGenMain(llvm::raw_ostream &OS, llvm::RecordKeeper &Records) {
   std::string Header = Indexer.FunctionToHeaderMap[FunctionName];
   auto RelPath = llvm::StringRef(Header).drop_back(2); // Drop the ".h" suffix.
   OS << "#include \"src/" << RelPath << "/" << FunctionName << ".h\"\n";
 
   auto &NameSpecPair = *Iter;
   llvm::Record *FunctionSpec = NameSpecPair.second;
   llvm::Record *RetValSpec = FunctionSpec->getValueAsDef("Return");
   llvm::Record *ReturnType = RetValSpec->getValueAsDef("ReturnType");
-  OS << "extern \"C\" " << Indexer.getTypeAsString(ReturnType) << " "
-     << FunctionName << "(";
+  std::string ReturnTypeString = Indexer.getTypeAsString(ReturnType);
+  bool ShouldReturn = true;
+  // We are generating C wrappers in C++ code. So, we should convert the C
+  // _Noreturn to the C++ [[noreturn]].
+  llvm::StringRef NR("_Noreturn "); // Note the space after _Noreturn
+  llvm::StringRef RT(ReturnTypeString);
+  if (RT.startswith(NR)) {
+    RT = RT.drop_front(NR.size() - 1); // - 1 because of the space.
+    ReturnTypeString = std::string("[[noreturn]]") + std::string(RT);
+    ShouldReturn = false;
+  }
+  OS << "extern \"C\" " << ReturnTypeString << " " << FunctionName << "(";
 
   auto ArgsList = FunctionSpec->getValueAsListOfDefs("Args");
   std::stringstream CallArgs;
   std::string ArgPrefix("__arg");
   for (size_t i = 0; i < ArgsList.size(); ++i) {
     llvm::Record *ArgType = ArgsList[i]->getValueAsDef("ArgType");
     auto TypeName = Indexer.getTypeAsString(ArgType);
 
if (i < ArgsList.size() - 1) {
       CallArgs << ", ";
     }
   }
 
   // TODO: Arg types of the C++ implementation functions need not
   // match the standard types. Either handle such differences here, or
   // avoid such a thing in the implementations.
   OS << ") {\n"
-     << "  return __llvm_libc::" << FunctionName << "(" << CallArgs.str()
-     << ");\n"
+     << "  " << (ShouldReturn ? "return " : "")
+     << "__llvm_libc::" << FunctionName << "(" << CallArgs.str() << ");\n"
      << "}\n";
 
   return false;
 }
 
 int main(int argc, char *argv[]) {
   llvm::cl::ParseCommandLineOptions(argc, argv);
   return TableGenMain(argv[0], WrapperGenMain);
 }