c++: constexpr, array, private ctor [PR120800]

c++: constexpr, array, private ctor [PR120800]

Here cxx_eval_vec_init_1 wants to recreate the default constructor call that
we previously built and threw away in build_vec_init_elt, but we aren't in
the same access context at this point.  Since we already checked access,
let's just suppress access control here.

Redoing overload resolution at constant evaluation time is sketchy, but
should usually be fine for a default/copy constructor.

	PR c++/120800

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_vec_init_1): Suppress access control.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-array30.C: New test.

(cherry picked from commit f6462f664725844faa97ae7e8690e4fedee65788)