[libc++][format] Granularizes the format header.

Moves the last pieces of code to its own header.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D133665

NOKEYCHECK=True
GitOrigin-RevId: 00798e50064447abd1483ff8e68a4dcaef7469d7
diff --git a/include/module.modulemap.in b/include/module.modulemap.in
index 969b6b5..b7f3380 100644
--- a/include/module.modulemap.in
+++ b/include/module.modulemap.in
@@ -661,10 +661,14 @@
         export locale
       }
       module format_error                    { private header "__format/format_error.h" }
+      module format_functions                { private header "__format/format_functions.h" }
       module format_fwd                      { private header "__format/format_fwd.h" }
       module format_parse_context            { private header "__format/format_parse_context.h" }
       module format_string                   { private header "__format/format_string.h" }
-      module format_to_n_result              { private header "__format/format_to_n_result.h" }
+      module format_to_n_result {
+        private header "__format/format_to_n_result.h"
+        export iterator.__iterator.incrementable_traits
+      }
       module formatter                       { private header "__format/formatter.h" }
       module formatter_bool                  { private header "__format/formatter_bool.h" }
       module formatter_char                  { private header "__format/formatter_char.h" }