c++: throw and private destructor [PR109172]

Since we aren't going through the normal call machinery, we need to check
the dtor access specifically.

	PR c++/109172

gcc/cp/ChangeLog:

	* except.cc (build_throw): Check dtor access.

gcc/testsuite/ChangeLog:

	* g++.dg/eh/dtor4.C: New test.