c++: block copy elision in delegating ctor

c++: block copy elision in delegating ctor

CWG2403 deals with the issue that copy elision is not possible when the
initialized object is a potentially-overlapping subobject and the
initializer is a function that returns by value.  Jonathan pointed out that
this also affects delegating constructors, which might be used to construct
a base subobject.

gcc/cp/ChangeLog:

	* call.cc (unsafe_return_slot_p): Return 2 for *this in a
	constructor.

gcc/testsuite/ChangeLog:

	* g++.dg/init/elide8.C: New test.