Remove use of scoped_restore_tmpl from scoped_restore_warning_hook

Remove use of scoped_restore_tmpl from scoped_restore_warning_hook

The warning_hook_handler function pointer takes va_list as
an argument, which on some platforms (mingw64) includes some
attributes. Attributes get ignored in template arguments.
This led to the compiler warning:

error: ignoring attributes on template argument 'warning_hook_handler' {aka 'void (*)(const char*, char*)'} [-Werror=ignored-attributes]
  387 |   scoped_restore_tmpl<warning_hook_handler> m_save;
      |                                           ^

By manually implementing the save/restore behaviour, rather
than using the helper template, this warning is fixed.

Approved-By: Tom Tromey <tom@tromey.com>