c++: inherited default constructor [CWG2799]

In this testcase, it seems clear that B should be trivially
default-constructible, since the inherited default constructor is trivial
and there are no other subobjects to initialize.  But we were saying no
because we don't define triviality of inherited constructors.

CWG discussion suggested that the solution is to implicitly declare a
default constructor when inheriting a default constructor; that makes sense
to me.

	DR 2799

gcc/cp/ChangeLog:

	* class.cc (add_implicit_default_ctor): Split out...
	(add_implicitly_declared_members): ...from here.
	Also call it when inheriting a default ctor.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/inh-ctor38.C: New test.