[Bug libstdc++/61166] overflow when parse number in std::duration operator""

[emsr at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61166

--- Comment #6 from emsr at gcc dot gnu.org ---
Something like parse_number was in the original doc as an implementation
example.  The idea was to select the smallest integral type that could
accommodate the number string with.  This is done with _Select_int. 
_Select_type picks the most space-efficient duration ratio type using
_Select_int.