include/fmt/core.h: fix building with clang-20

Signed-off-by: Denis Pronin <dannftk@yandex.ru>

Author: dpronin

include/fmt/core.h

@@ -2709,7 +2709,7 @@ template <typename Char, typename... Args> class basic_format_string {
   template <typename S,
             FMT_ENABLE_IF(
                 std::is_convertible<const S&, basic_string_view<Char>>::value)>
-  FMT_CONSTEVAL FMT_INLINE basic_format_string(const S& s) : str_(s) {
+  FMT_CONSTEXPR FMT_INLINE basic_format_string(const S& s) : str_(s) {
     static_assert(
         detail::count<
             (std::is_base_of<detail::view, remove_reference_t<Args>>::value &&