[libc++][format] Add __format_arg_store.

This implements the struct `__format_arg_store` and its dependencies:
* the class basic_format_arg,
* the class basic_format_args,
* the class basic_format_context,
* the function make_format_args,
* the function wmake_format_args,
* the function visit_format_arg,
* several Standard required typedefs.

The following parts will be implemented in a later patch:

* the child class `basic_format_arg::handle`,
* the function `basic_format_arg::basic_format_arg(const T* p)`.

The following extension has been implemented:
* the class basic_format_arg supports `__[u]int128_t` on platform where libc++ supports 128 bit integrals.

Implements parts of:
* P0645 Text Formatting

Completes:
* LWG3371 visit_format_arg and make_format_args are not hidden friends
* LWG3542 basic_format_arg mishandles basic_string_view with custom traits

Note https://mordante.github.io/blog/2021/06/05/format.html gives a bit more information about the goals and non-goals of this initial patch series.

Reviewed By: #libc, ldionne, vitaut

Differential Revision: https://reviews.llvm.org/D103357

NOKEYCHECK=True
GitOrigin-RevId: 0922ce56f4f04fbcacead6cdf0416341fe44e4bb
diff --git a/include/module.modulemap b/include/module.modulemap
index f0d6023..5ef0f9a 100644
--- a/include/module.modulemap
+++ b/include/module.modulemap
@@ -431,7 +431,15 @@
     export *
 
     module __format {
+      module format_arg           { private header "__format/format_arg.h"           }
+      module format_args          { private header "__format/format_args.h"          }
+      module format_context {
+        private header "__format/format_context.h"
+        export optional
+        export locale
+      }
       module format_error         { private header "__format/format_error.h"         }
+      module format_fwd           { private header "__format/format_fwd.h"           }
       module format_parse_context { private header "__format/format_parse_context.h" }
     }
   }