libstdc++: Make std::vector<bool>::reference constructor private [PR115098]

libstdc++: Make std::vector<bool>::reference constructor private [PR115098]

The standard says this constructor should be private.  LWG 4141 proposes
to remove it entirely. We still need it, but it doesn't need to be
public.

For std::bitset the default constructor is already private (and never
even defined) but there's a non-standard constructor that's public, but
doesn't need to be.

libstdc++-v3/ChangeLog:

	PR libstdc++/115098
	* include/bits/stl_bvector.h (_Bit_reference): Make default
	constructor private. Declare vector and bit iterators as
	friends.
	* include/std/bitset (bitset::reference): Make constructor and
	data members private.
	* testsuite/20_util/bitset/115098.cc: New test.
	* testsuite/23_containers/vector/bool/115098.cc: New test.