[clang] [libcxx] [Clang] Add __common_type builtin (PR #99473)

[A. Jiang via cfe-commits]

================
@@ -1511,6 +1511,43 @@ Attributes (N2335)                                                            C2
 ``#embed`` (N3017)                                                            C23           C89, C++
 ============================================ ================================ ============= =============
 
+Builtin type aliases
+====================
+
+Clang provides a few builtin aliases to improve the throughput of certain metaprogramming facilities.
+
+__common_type
+-------------
+
+.. code-block:: c++
+  template <template <class... Args> class BaseTemplate,
+            template <class TypeMember> class HasTypeMember,
+            class HasNoTypeMember,
+            class... Ts>
+  using __common_type = ...;
+
+This alias is used for implementing ``std::common_type``. If ``std::common_type`` should contain a ``type`` member,
+it is an alias to ``HasTypeMember<TheCommonType>``. Otherwise it is an alias to ``HasNoTypeMember``. The
+``BaseTemplate`` is usually ``std::common_type``. ``Ts`` are the arguments to ``std::common_type``.
+
+__type_pack_element
+-------------------
+
+.. code-block:: c++
+  template <std::size_t Index, class... Ts>
+  using __type_pack_element = ...;
+
+This alias returns the type at ``Index`` in the parameter pack ``Ts``.
+
+__make_integer_seq
+------------------
+
+.. code-block:: c++
+  template <template <class IntSeqT, IntSeqT... Ints> class IntSeq, class T, class N>
----------------
frederick-vs-ja wrote:

```suggestion
  template <template <class IntSeqT, IntSeqT... Ints> class IntSeq, class T, T N>
```
Typo?

https://github.com/llvm/llvm-project/pull/99473