[Lex] Add __has_builtin support for __make_integer_seq

lib/Lex/PPMacroExpansion.cpp

if (II == Ident__LINE__) {
 
     int Value = 0;
     if (!IsValid)
       Diag(StartLoc, diag::err_feature_check_malformed);
     else if (II == Ident__is_identifier)
       Value = FeatureII->getTokenID() == tok::identifier;
     else if (II == Ident__has_builtin) {
       // Check for a builtin is trivial.
-      Value = FeatureII->getBuiltinID() != 0;
+      if (FeatureII->getBuiltinID() != 0) {
+        Value = true;
+      } else {
+        const LangOptions &LangOpts = PP.getLangOpts();
+        StringRef Feature = FeatureII->getName();
+        Value = llvm::StringSwitch<bool>(Feature)
+                    .Case("__make_integer_seq", LangOpts.CPlusPlus)
+                    .Default(false);
+      }
     } else if (II == Ident__has_attribute)
       Value = hasAttribute(AttrSyntax::GNU, nullptr, FeatureII,
                            getTargetInfo(), getLangOpts());
     else if (II == Ident__has_cpp_attribute)
       Value = hasAttribute(AttrSyntax::CXX, ScopeII, FeatureII,
                            getTargetInfo(), getLangOpts());
     else if (II == Ident__has_declspec)
       Value = hasAttribute(AttrSyntax::Declspec, nullptr, FeatureII,

test/SemaCXX/make_integer_seq.cpp

 // RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 %s
 
+static_assert(__has_builtin(__make_integer_seq), "");
+
 template <class T, T... I>
 struct Seq {
   static constexpr T PackSize = sizeof...(I);
 };
 
 template <typename T, T N>
 using MakeSeq = __make_integer_seq<Seq, T, N>;