[libc++] Annotate generic_category/system_category as const (#78052)

This lets the compiler eliminate unused/duplicate calls to it.

Author: d0k

libcxx/include/__system_error/error_category.h

@@ -67,8 +67,8 @@ class _LIBCPP_HIDDEN __do_message : public error_category {
   string message(int __ev) const override;
 };
 
-_LIBCPP_EXPORTED_FROM_ABI const error_category& generic_category() _NOEXCEPT;
-_LIBCPP_EXPORTED_FROM_ABI const error_category& system_category() _NOEXCEPT;
+__attribute__((__const__)) _LIBCPP_EXPORTED_FROM_ABI const error_category& generic_category() _NOEXCEPT;
+__attribute__((__const__)) _LIBCPP_EXPORTED_FROM_ABI const error_category& system_category() _NOEXCEPT;
 
 _LIBCPP_END_NAMESPACE_STD