[clang] Update __cpp_concepts macro (PR #87998)

[Aaron Ballman via cfe-commits]

================
@@ -110,6 +110,11 @@ C++20 Feature Support
   templates (`P1814R0 <https://wg21.link/p1814r0>`_).
   (#GH54051).
 
+- __cpp_concepts macro now updated to `202002L`, as we are confident that,
+  modulo a handful of bugs and core issues, that our concepts implementation is
+  sufficient. This should enable libstdc++'s `<expected>` to work correctly in
+  clang.
----------------
AaronBallman wrote:

```suggestion
- We have sufficient confidence in the concepts implementation to update the
  ``__cpp_concepts`` macro to `202002L`. This enables ``<expected>`` from
  libstdc++ to work correctly with Clang.
```
??

https://github.com/llvm/llvm-project/pull/87998