libstdc++: Replace _Hashtable::__fwd_value_for with cast

We can just use a cast to the appropriate type instead of calling a
function to do it. This gives the compiler less work to compile and
optimize, and at -O0 avoids a function call per element.

libstdc++-v3/ChangeLog:

	* include/bits/hashtable.h (_Hashtable::__fwd_value_for):
	Remove.
	(_Hashtable::_M_assign): Use static_cast instead of
	__fwd_value_for.

Reviewed-by: François Dumont <fdumont@gcc.gnu.org>