[libc++][NFC] Move format_to_n_result.

Places `format_to_n_result` to its own file. While working on D112361 it
turns out the type will be used outside the format header.

Reviewed By: #libc, Quuxplusone, Mordante

Differential Revision: https://reviews.llvm.org/D113831

NOKEYCHECK=True
GitOrigin-RevId: 5baa4ee30b5c2d3920499d974c152525636ab8ac
diff --git a/include/module.modulemap b/include/module.modulemap
index 39b8a66..b159d58 100644
--- a/include/module.modulemap
+++ b/include/module.modulemap
@@ -463,6 +463,7 @@
       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 formatter              { private header "__format/formatter.h"              }
       module formatter_bool         { private header "__format/formatter_bool.h"         }
       module formatter_char         { private header "__format/formatter_char.h"         }