[ItaniumMangle] Correctly mangle BuiltinTemplateDecls

A BuiltinTemplateDecl has no underlying templated decl and as such they
cannot be relied upon for mangling.  The ItaniumMangler had some bugs
here which lead to crashes.

This fixes PR28519.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@275190 91177308-0d34-0410-b5e6-96231b3b80d8

lib/AST/ItaniumMangle.cpp

@@ -910,6 +910,8 @@ void CXXNameMangler::mangleUnscopedTemplateName(
     assert(!AdditionalAbiTags &&
            "template template param cannot have abi tags");
     mangleTemplateParameter(TTP->getIndex());
+  } else if (isa<BuiltinTemplateDecl>(ND)) {
+    mangleUnscopedName(ND, AdditionalAbiTags);
   } else {
     mangleUnscopedName(ND->getTemplatedDecl(), AdditionalAbiTags);
   }
@@ -1715,7 +1717,10 @@ void CXXNameMangler::mangleTemplatePrefix(const TemplateDecl *ND,
     mangleTemplateParameter(TTP->getIndex());
   } else {
     manglePrefix(getEffectiveDeclContext(ND), NoFunction);
-    mangleUnqualifiedName(ND->getTemplatedDecl(), nullptr);
+    if (isa<BuiltinTemplateDecl>(ND))
+      mangleUnqualifiedName(ND, nullptr);
+    else
+      mangleUnqualifiedName(ND->getTemplatedDecl(), nullptr);
   }
 
   addSubstitution(ND);

test/CodeGenCXX/mangle-template.cpp

@@ -201,3 +201,14 @@ namespace test14 {
 
   int call(bool b) { return inl<void>(b); }
 }
+
+namespace std {
+template <class _Tp, _Tp...> struct integer_sequence {};
+}
+
+namespace test15 {
+template <int N>
+__make_integer_seq<std::integer_sequence, int, N> make() {}
+template __make_integer_seq<std::integer_sequence, int, 5> make<5>();
+// CHECK: define weak_odr void @_ZN6test154makeILi5EEE18__make_integer_seqISt16integer_sequenceiXT_EEv(
+}