[libc++][format] LWG4061: Should std::basic_format_context be default-constructible/copyable/movable? (#97251)

See [LWG4061](https://cplusplus.github.io/LWG/issue4061) and
[P3341R0](https://wg21.link/p3341r0). Effectively reverts commit
36ce0c3b1e581ca310ae7d0cbc6af002cc5d0251.


`libcxx/test/std/utilities/format/format.functions/bug_81590.compile.pass.cpp`
has a `format` function that unexpectedly takes the
`basic_format_context` by value, which is made ill-formed by LWG4061.
This PR changes the function to take the context by reference.

libcxx/docs/Status/Cxx2cIssues.csv

@@ -64,7 +64,7 @@
 "","","","","",""
 "`3944 <https://wg21.link/LWG3944>`__","Formatters converting sequences of ``char`` to sequences of ``wchar_t``","St. Louis June 2024","","","|format|"
 "`4060 <https://wg21.link/LWG4060>`__","``submdspan`` preconditions do not forbid creating invalid pointer","St. Louis June 2024","","",""
-"`4061 <https://wg21.link/LWG4061>`__","Should ``std::basic_format_context`` be default-constructible/copyable/movable?","St. Louis June 2024","","","|format|"
+"`4061 <https://wg21.link/LWG4061>`__","Should ``std::basic_format_context`` be default-constructible/copyable/movable?","St. Louis June 2024","|Complete|","19.0","|format|"
 "`4071 <https://wg21.link/LWG4071>`__","``reference_wrapper`` comparisons are not SFINAE-friendly","St. Louis June 2024","|Complete|","19.0",""
 "`4074 <https://wg21.link/LWG4074>`__","``compatible-joinable-ranges`` is underconstrained","St. Louis June 2024","","","|ranges|"
 "`4076 <https://wg21.link/LWG4076>`__","``concat_view`` should be freestanding","St. Louis June 2024","","",""

libcxx/include/__format/format_context.h

@@ -131,6 +131,9 @@ private:
   _LIBCPP_HIDE_FROM_ABI explicit basic_format_context(_OutIt __out_it, basic_format_args<basic_format_context> __args)
       : __out_it_(std::move(__out_it)), __args_(__args) {}
 #  endif
+
+  basic_format_context(const basic_format_context&)            = delete;
+  basic_format_context& operator=(const basic_format_context&) = delete;
 };
 
 // A specialization for __retarget_buffer

libcxx/test/std/utilities/format/format.formatter/format.context/format.context/ctor.pass.cpp

@@ -112,19 +112,27 @@ void test() {
 #endif
 }
 
-// std::back_insert_iterator<std::string>, copyable
-static_assert(std::is_copy_constructible_v<std::basic_format_context<std::back_insert_iterator<std::string>, char>>);
-static_assert(std::is_copy_assignable_v<std::basic_format_context<std::back_insert_iterator<std::string>, char>>);
+// The default constructor is suppressed by the deleted copy operations.
+// The move operations are implicitly deleted due to the deleted copy operations.
 
-static_assert(std::is_move_constructible_v<std::basic_format_context<std::back_insert_iterator<std::string>, char>>);
-static_assert(std::is_move_assignable_v<std::basic_format_context<std::back_insert_iterator<std::string>, char>>);
+// std::back_insert_iterator<std::string>, copyable
+static_assert(
+    !std::is_default_constructible_v<std::basic_format_context<std::back_insert_iterator<std::string>, char>>);
+
+static_assert(!std::is_copy_constructible_v<std::basic_format_context<std::back_insert_iterator<std::string>, char>>);
+static_assert(!std::is_copy_assignable_v<std::basic_format_context<std::back_insert_iterator<std::string>, char>>);
+
+static_assert(!std::is_move_constructible_v<std::basic_format_context<std::back_insert_iterator<std::string>, char>>);
+static_assert(!std::is_move_assignable_v<std::basic_format_context<std::back_insert_iterator<std::string>, char>>);
 
 // cpp20_output_iterator, move only
+static_assert(!std::is_default_constructible_v<std::basic_format_context<cpp20_output_iterator<int*>, char>>);
+
 static_assert(!std::is_copy_constructible_v<std::basic_format_context<cpp20_output_iterator<int*>, char>>);
 static_assert(!std::is_copy_assignable_v<std::basic_format_context<cpp20_output_iterator<int*>, char>>);
 
-static_assert(std::is_move_constructible_v<std::basic_format_context<cpp20_output_iterator<int*>, char>>);
-static_assert(std::is_move_assignable_v<std::basic_format_context<cpp20_output_iterator<int*>, char>>);
+static_assert(!std::is_move_constructible_v<std::basic_format_context<cpp20_output_iterator<int*>, char>>);
+static_assert(!std::is_move_assignable_v<std::basic_format_context<cpp20_output_iterator<int*>, char>>);
 
 int main(int, char**) {
   test<std::back_insert_iterator<std::basic_string<char>>, char>();

libcxx/test/std/utilities/format/format.functions/bug_81590.compile.pass.cpp

@@ -25,7 +25,7 @@ struct X : std::variant<X*> {
 
 template <>
 struct std::formatter<X, char> : std::formatter<std::string, char> {
-  static constexpr auto format(const X& x, auto ctx) {
+  static constexpr auto format(const X& x, auto& ctx) {
     if (!x.p)
       return ctx.out();
     auto m = [&](const X* t) { return std::format_to(ctx.out(), "{}", *t); };