libstdc++: Implement P2918R0 "Runtime format strings II" for C++26

This adds std::runtime_format for C++26. These new overloaded functions
enhance the std::format API so that it isn't necessary to use the less
ergonomic std::vformat and std::make_format_args (which are meant to be
implementation details). This was approved in Kona 2023 for C++26.

libstdc++-v3/ChangeLog:

	* include/std/format (__format::_Runtime_format_string): Define
	new class template.
	(basic_format_string): Add non-consteval constructor for runtime
	format strings.
	(runtime_format): Define new function for C++26.
	* testsuite/std/format/runtime_format.cc: New test.

libstdc++-v3/include/std/format

@@ -81,6 +81,9 @@ namespace __format
 
   template<typename _CharT>
     using __format_context = basic_format_context<_Sink_iter<_CharT>, _CharT>;
+
+  template<typename _CharT>
+    struct _Runtime_format_string { basic_string_view<_CharT> _M_str; };
 } // namespace __format
 /// @endcond
 
@@ -115,6 +118,11 @@ namespace __format
 	consteval
 	basic_format_string(const _Tp& __s);
 
+      [[__gnu__::__always_inline__]]
+      basic_format_string(__format::_Runtime_format_string<_CharT>&& __s)
+      : _M_str(__s._M_str)
+      { }
+
       [[__gnu__::__always_inline__]]
       constexpr basic_string_view<_CharT>
       get() const noexcept
@@ -133,6 +141,20 @@ namespace __format
       = basic_format_string<wchar_t, type_identity_t<_Args>...>;
 #endif
 
+#if __cplusplus > 202302L
+  [[__gnu__::__always_inline__]]
+  inline __format::_Runtime_format_string<char>
+  runtime_format(string_view __fmt)
+  { return {__fmt}; }
+
+#ifdef _GLIBCXX_USE_WCHAR_T
+  [[__gnu__::__always_inline__]]
+  inline __format::_Runtime_format_string<wchar_t>
+  runtime_format(wstring_view __fmt)
+  { return {__fmt}; }
+#endif
+#endif // C++26
+
   // [format.formatter], formatter
 
   /// The primary template of std::formatter is disabled.

libstdc++-v3/testsuite/std/format/runtime_format.cc

+// { dg-do run { target c++26 } }
+
+#include <format>
+#include <testsuite_hooks.h>
+
+void
+test_char()
+{
+  std::string fmt = "{}";
+  auto s = std::format(std::runtime_format(fmt), 123);
+  VERIFY( s == "123" );
+}
+
+void
+test_wchar()
+{
+  std::wstring fmt = L"{:#o}";
+  auto s = std::format(std::runtime_format(fmt), 456);
+  VERIFY( s == L"0710" );
+}
+
+void
+test_internal_api()
+{
+  // Using _Runtime_format_string directly works even in C++20 mode.
+  // This can be used internally by libstdc++.
+  std::string fmt = "{:#x}";
+  auto s = std::format(std::__format::_Runtime_format_string<char>(fmt), 789);
+  VERIFY( s == "0x315" );
+}
+
+int main()
+{
+  test_char();
+  test_wchar();
+  test_internal_api();
+}