[libc++][format] Adds formatter<charT[N], charT>.

This formatter isn't in the list of required formatters in

[format.formatter.spec]/2.2
  For each charT, the string type specializations
   template<> struct formatter<charT*, charT>;
  template<> struct formatter<const charT*, charT>;
  template<size_t N> struct formatter<const charT[N], charT>;
  template<class traits, class Allocator>
    struct formatter<basic_string<charT, traits, Allocator>, charT>;
  template<class traits>
    struct formatter<basic_string_view<charT, traits>, charT>;

Since remove_cvref_t<const charT[N]> is charT[N] the formatter is
required by

[format.functions]/25
  Preconditions: formatter<remove_cvref_t<Ti>, charT> meets the
  BasicFormatter requirements ([formatter.requirements]) for each Ti in
  Args.

Depends on D120921

Reviewed By: #libc, Mordante

Differential Revision: https://reviews.llvm.org/D121138