libstdc++: Simplify __detail::__distance_fw using 'if constexpr'

libstdc++: Simplify __detail::__distance_fw using 'if constexpr'

This uses 'if constexpr' instead of tag dispatching, removing the need
for a second call using that tag, and simplifying the overload set that
needs to be resolved for calls to __distance_fw.

libstdc++-v3/ChangeLog:

	* include/bits/hashtable_policy.h (__distance_fw): Replace tag
	dispatching with 'if constexpr'.