clover needs to walk valid specifiers in C++, vtn needs
to find string specifiers in C, let's do both.
This writes the format walker in C++, and wraps it with C.
Acked-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8254>
