[Tooling/Inclusion] Make the Recognizer work for C99 code.

[hokein]

Summary

C99 is the the earliest C version provided by the language opt.

clang/lib/Tooling/Inclusions/Stdlib/StandardLibrary.cpp

NSSymbolMap *Result = [&]() -> NSSymbolMap * {
     return getMappingPerLang(L)->NamespaceSymbols->lookup(Scope);
   }();
   NamespaceCache.try_emplace(D, Result);
   return Result;
 }
 
 std::optional<Symbol> Recognizer::operator()(const Decl *D) {
   Lang L;
-  if (D->getLangOpts().CPlusPlus) {
+  if (D->getLangOpts().CPlusPlus)
     L = Lang::CXX;
-  } else if (D->getLangOpts().C11) {
+  else if (D->getLangOpts().C99)
     L = Lang::C;
-  } else {
+  else
     return std::nullopt; // not a supported language.
-  }
 
   // If D is std::vector::iterator, `vector` is the outer symbol to look up.
   // We keep all the candidate DCs as some may turn out to be anon enums.
   // Do this resolution lazily as we may turn out not to have a std namespace.
   llvm::SmallVector<const DeclContext *> IntermediateDecl;
   const DeclContext *DC = D->getDeclContext();
   if (!DC) // The passed D is a TranslationUnitDecl!
     return std::nullopt;

clang/unittests/Tooling/StandardLibraryTest.cpp

TEST(StdlibTest, Recognizer) {
   EXPECT_EQ(Recognizer(Clock),
             stdlib::Symbol::named("std::chrono::", "system_clock"));
   auto DivT = stdlib::Symbol::named("", "div_t", stdlib::Lang::CXX);
   EXPECT_TRUE(DivT);
   EXPECT_EQ(Recognizer(CDivT), DivT);
   EXPECT_EQ(Recognizer(Sec), std::nullopt);
 }
 
+TEST(StdlibTest, RecognizerForC99) {
+  TestInputs Input("typedef char uint8_t;");
+  Input.Language = TestLanguage::Lang_C99;
+  TestAST AST(Input);
+
+  auto &Uint8T = lookup(AST, "uint8_t");
+  stdlib::Recognizer Recognizer;
+  EXPECT_EQ(Recognizer(&Uint8T),
+            stdlib::Symbol::named("", "uint8_t", stdlib::Lang::C));
+}
+
 } // namespace
 } // namespace tooling
 } // namespace clang