libstdc++: Fix return value of vector::insert_range

In some cases we're wrongly returning an iterator to (one past) the last
element inserted instead of to the first element inserted.

libstdc++-v3/ChangeLog:

	* include/bits/stl_bvector.h (vector<bool>::insert_range):
	Consistently return an iterator pointing to the first element
	inserted.
	* include/bits/vector.tcc (vector::insert_range): Likewise.
	* testsuite/23_containers/vector/bool/modifiers/insert/insert_range.cc:
	Verify insert_range return values.
	* testsuite/23_containers/vector/modifiers/insert/insert_range.cc:
	Likewise.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>