Requires: Facet is a facet class whose definition contains the public static member id as defined in [locale.facet].
Returns: A reference to the corresponding facet of loc, if present.
Throws: bad_cast if has_facet<Facet>(loc) is false.
Remarks: The reference returned remains valid at least as long as any copy of loc exists.
Returns: True if the facet requested is present in loc; otherwise false.