[NFC][libc++][format] Switches to from_range constructor.

[Mordante]

Summary

Some places in the format library were identified to benefit from
basic_string's from_range constructor. At that time that constructor was
not implemented. It's implemented now so adjust the code to use this new
constructor.

[var-const]

Looks cool!

libcxx/include/__format/range_default_formatter.h

 #include <__concepts/same_as.h>
 #include <__config>
 #include <__format/concepts.h>
 #include <__format/formatter.h>
 #include <__format/range_formatter.h>
 #include <__iterator/back_insert_iterator.h>
 #include <__ranges/concepts.h>
 #include <__ranges/data.h>
+#include <__ranges/from_range.h>
 #include <__ranges/size.h>
 #include <__type_traits/conditional.h>
 #include <__type_traits/remove_cvref.h>
 #include <__utility/pair.h>
 #include <string_view>
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
public:
   template <class _FormatContext>
   _LIBCPP_HIDE_FROM_ABI typename _FormatContext::iterator
   format(conditional_t<ranges::input_range<const _Rp>, const _Rp&, _Rp&> __range, _FormatContext& __ctx) const {
     // When the range is contiguous use a basic_string_view instead to avoid a
     // copy of the underlying data. The basic_string_view formatter
     // specialization is the "basic" string formatter in libc++.
     if constexpr (ranges::contiguous_range<_Rp> && std::ranges::sized_range<_Rp>)
       return __underlying_.format(basic_string_view<_CharT>{ranges::data(__range), ranges::size(__range)}, __ctx);
-    else {
-      // P2106's from_range has not been implemented yet. Instead use a simple
-      // copy operation.
-      // TODO FMT use basic_string's "from_range" constructor.
-      // return __underlying_.format(basic_string<_CharT>{from_range, __range}, __ctx);
-      basic_string<_CharT> __str;
-      std::ranges::copy(__range, back_insert_iterator{__str});
-      return __underlying_.format(static_cast<basic_string_view<_CharT>>(__str), __ctx);
-    }
+    else
+      return __underlying_.format(basic_string<_CharT>{from_range, __range}, __ctx);
   }
 };
 
 template <ranges::input_range _Rp, class _CharT>
   requires(format_kind<_Rp> != range_format::disabled && formattable<ranges::range_reference_t<_Rp>, _CharT>)
 struct _LIBCPP_TEMPLATE_VIS formatter<_Rp, _CharT> : __range_default_formatter<format_kind<_Rp>, _Rp, _CharT> {};
 
 #endif //_LIBCPP_STD_VER >= 23
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___FORMAT_RANGE_DEFAULT_FORMATTER_H

libcxx/include/__format/range_formatter.h

 #include <__format/format_context.h>
 #include <__format/format_error.h>
 #include <__format/formatter.h>
 #include <__format/formatter_output.h>
 #include <__format/parser_std_format_spec.h>
 #include <__iterator/back_insert_iterator.h>
 #include <__ranges/concepts.h>
 #include <__ranges/data.h>
+#include <__ranges/from_range.h>
 #include <__ranges/size.h>
 #include <__type_traits/remove_cvref.h>
 #include <string_view>
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER >= 23
 
if constexpr (ranges::contiguous_range<_Rp> && std::ranges::sized_range<_Rp>) {
               ranges::data(__range),
               ranges::size(__range),
           },
           __ctx);
     } else {
       std::formatter<basic_string<_CharT>, _CharT> __formatter;
       if (__debug_format)
         __formatter.set_debug_format();
-      // P2106's from_range has not been implemented yet. Instead use a simple
-      // copy operation.
-      // TODO FMT use basic_string's "from_range" constructor.
-      // return std::formatter<basic_string<_CharT>, _CharT>{}.format(basic_string<_CharT>{from_range, __range}, __ctx);
-      basic_string<_CharT> __str;
-      ranges::copy(__range, back_insert_iterator{__str});
-      return __formatter.format(__str, __ctx);
+      return __formatter.format(basic_string<_CharT>{from_range, __range}, __ctx);
     }
   }
 
   template <ranges::input_range _Rp, class _FormatContext>
   _LIBCPP_HIDE_FROM_ABI typename _FormatContext::iterator
   __format_as_sequence(_Rp&& __range, _FormatContext& __ctx) const {
     __ctx.advance_to(ranges::copy(__opening_bracket_, __ctx.out()).out);
     bool __use_separator = false;