libstdc++: Simplify std::vector::vector(from_range_t, const Alloc&)

libstdc++: Simplify std::vector::vector(from_range_t, const Alloc&)

Tomasz suggested replacing this constructor with just append_range(rg),
after using a delegating constructor so that the destructor will run if
append_range exits via an exception.

This is slightly less simple than his suggestion, because I want to
avoid the overhead of reserve's slow path and the ASan annotations.
Neither of those is needed for this constructor, because we have no
existing storage to reallocate and no unused capacity to tell ASan
about.

libstdc++-v3/ChangeLog:

	* include/bits/stl_vector.h (vector(from_range_t, Alloc)): Use
	delegating constructor instead of RAII guards. Use append_range
	for unsized input range case.

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