c++: constexpr empty aggr [PR105795]

c++: constexpr empty aggr [PR105795]

In this testcase, leaving ctx->ctor pointing to the enclosing object meant
that evaluating the initializer for the subobject clobbered previous
initializers for the enclosing object.  So do update ctx->ctor, just don't
add it to the enclosing object ctor.

	PR c++/105795

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_bare_aggregate): Always call
	init_subob_ctx.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/constexpr-aggr-base1.C: New test.