[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/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index 806b30f..8c4f2a8 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -198,7 +198,7 @@
     ------------------------------------------------- -----------------
     ``__cpp_lib_char8_t``                             ``201811L``
     ------------------------------------------------- -----------------
-    ``__cpp_lib_concepts``                            *unimplemented*
+    ``__cpp_lib_concepts``                            ``202002L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_algorithms``                ``201806L``
     ------------------------------------------------- -----------------