[libcxx] adds __cpp_lib_concepts feature-test macro

Also adjusts C++20 status paper to indicate full concepts support.

Depends on D96477, D99817.

Differential Revision: https://reviews.llvm.org/D99805

GitOrigin-RevId: 920c0f7e09e2a55a800a181fb9bfe296a8733c89
diff --git a/include/concepts b/include/concepts
index 6ce2fad..7127f28 100644
--- a/include/concepts
+++ b/include/concepts
@@ -67,9 +67,9 @@
   template<class T, class... Args>
     concept constructible_from = see below;
 
-  // [concept.defaultconstructible], concept default_constructible
+  // [concept.default.init], concept default_initializable
   template<class T>
-    concept default_constructible = see below;
+    concept default_initializable = see below;
 
   // [concept.moveconstructible], concept move_constructible
   template<class T>
@@ -79,11 +79,6 @@
   template<class T>
     concept copy_constructible = see below;
 
-  // [concepts.compare], comparison concepts
-  // [concept.boolean], concept boolean
-  template<class B>
-    concept boolean = see below;
-
   // [concept.equalitycomparable], concept equality_comparable
   template<class T>
     concept equality_comparable = see below;