Tag official API for module export (#2235)

* functions
 * classes
 * UDLs
 * other declarations

Export everything in namespace 'fmt' from core.h and format.h

include/fmt/printf.h

@@ -194,6 +194,7 @@ FMT_DEPRECATED void printf(detail::buffer<Char>& buf,
 }
 using detail::vprintf;
 
+// already exported through "ostream.h" above
 template <typename Char>
 class basic_printf_parse_context : public basic_format_parse_context<Char> {
   using basic_format_parse_context<Char>::basic_format_parse_context;
@@ -567,6 +568,8 @@ OutputIt basic_printf_context<OutputIt, Char>::format() {
       out, basic_string_view<Char>(start, detail::to_unsigned(it - start)));
 }
 
+FMT_MODULE_EXPORT_BEGIN
+
 template <typename Char>
 using basic_printf_context_t =
     basic_printf_context<detail::buffer_appender<Char>, Char>;
@@ -717,6 +720,8 @@ inline int fprintf(std::basic_ostream<Char>& os, const S& format_str,
   return vfprintf(os, to_string_view(format_str),
                   make_format_args<context>(args...));
 }
+
+FMT_MODULE_EXPORT_END
 FMT_END_NAMESPACE
 
 #endif  // FMT_PRINTF_H_