[ADT] Add makeIntrusiveRefCnt helper function

[njames93]

Summary

Works like std::make_unique but for IntrusiveRefCntPtr objects.
See https://lists.llvm.org/pipermail/llvm-dev/2021-January/147729.html

[njames93]

llvm/unittests/ADT/IntrusiveRefCntPtrTest.cpp

62–67

Couldn't think how best to test this, but these seem to do a good enough job.

[dblaikie]

Looks good - thanks!

[MaskRay]

Looks great!

llvm/include/llvm/ADT/IntrusiveRefCntPtr.h

300

s/smart/intrusive/.

Though I guess "smart ref counted pointers" can just be omitted.

[njames93]

Address comments.

llvm/include/llvm/ADT/IntrusiveRefCntPtr.h

 template <class T> struct simplify_type<const IntrusiveRefCntPtr<T>> {
   using SimpleType = /*const*/ T *;
 
   static SimpleType getSimplifiedValue(const IntrusiveRefCntPtr<T> &Val) {
     return Val.get();
   }
 };
 
+/// Factory function for creating smart ref counted pointers.
+template <typename T, typename... Args>
+IntrusiveRefCntPtr<T> makeIntrusiveRefCnt(Args &&...A) {
+  return IntrusiveRefCntPtr<T>(new T(std::forward<Args>(A)...));
+}
+
 } // end namespace llvm
 
 #endif // LLVM_ADT_INTRUSIVEREFCNTPTR_H

llvm/unittests/ADT/IntrusiveRefCntPtrTest.cpp

EXPECT_EQ(0, NumInstances);
     auto S1 = std::make_unique<TypeParam>();
     IntrusiveRefCntPtr<TypeParam> R1 = std::move(S1);
     EXPECT_EQ(1, NumInstances);
     EXPECT_EQ(S1, nullptr);
   }
   EXPECT_EQ(0, NumInstances);
 }
 
+TYPED_TEST(IntrusiveRefCntPtrTest, MakeIntrusive) {
+  EXPECT_EQ(0, NumInstances);
+  {
+    auto S1 = makeIntrusiveRefCnt<TypeParam>();
+    auto S2 = makeIntrusiveRefCnt<const TypeParam>();
+    EXPECT_EQ(2, NumInstances);
+    static_assert(
+        std::is_same<decltype(S1), IntrusiveRefCntPtr<TypeParam>>::value,
+        "Non-const type mismatch");
+    static_assert(
+        std::is_same<decltype(S2), IntrusiveRefCntPtr<const TypeParam>>::value,
+        "Const type mismatch");
+  }
+  EXPECT_EQ(0, NumInstances);
+}
+
 struct InterceptRefCounted : public RefCountedBase<InterceptRefCounted> {
   InterceptRefCounted(bool *Released, bool *Retained)
     : Released(Released), Retained(Retained) {}
   bool * const Released;
   bool * const Retained;
 };
 template <> struct IntrusiveRefCntPtrInfo<InterceptRefCounted> {
   static void retain(InterceptRefCounted *I) {