[libc++][format] Adds a formattable concept.

The concept is based on P2286R2 Formatting Ranges. It will be used to
optimise the storage of __format_arg_store as required by LWG-3473.

Depends on D120916

Reviewed By: #libc, Mordante

Differential Revision: https://reviews.llvm.org/D120921

NOKEYCHECK=True
GitOrigin-RevId: 15c809e8e78083d59f1c0b09ca1d1644e0976961
diff --git a/include/module.modulemap b/include/module.modulemap
index 7ae3d10..ef97b81 100644
--- a/include/module.modulemap
+++ b/include/module.modulemap
@@ -542,6 +542,7 @@
 
     module __format {
       module buffer                   { private header "__format/buffer.h" }
+      module concepts                 { private header "__format/concepts.h" }
       module enable_insertable        { private header "__format/enable_insertable.h" }
       module format_arg               { private header "__format/format_arg.h" }
       module format_args              { private header "__format/format_args.h" }