libstdc++: Replace some implicit conversions in std::vector

libstdc++: Replace some implicit conversions in std::vector

This replaces two implicit conversions from ptrdiff_t to size_t with
explicit conversions that include unreachable hints for the ptrdiff_t
value not being negative.

libstdc++-v3/ChangeLog:

	* include/bits/stl_vector.h (~_Vector_base): Add unreachable
	hint for negative capacity and cast to size_t explicitly.
	* include/bits/vector.tcc (vector::_M_realloc_append): Use
	size() instead of end() - begin().

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>