[libc++] Granularize <concept> includes

Reviewed By: ldionne, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D137283

NOKEYCHECK=True
GitOrigin-RevId: 89b356f05ab7aa3d96fc7b68aece6e7a5bdb0db5
diff --git a/include/regex b/include/regex
index 91a10fe..8118c94 100644
--- a/include/regex
+++ b/include/regex
@@ -6858,6 +6858,7 @@
 _LIBCPP_POP_MACROS
 
 #if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
+#  include <concepts>
 #  include <iterator>
 #  include <utility>
 #endif