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

[Aaron Ballman via cfe-commits]

================
@@ -1511,6 +1511,46 @@ 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.
+
+__builtin_common_type
+-------------
----------------
AaronBallman wrote:

```suggestion
__builtin_common_type
---------------------
```

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