C++ PATCH for c++/71913 (copy elision choices)

[Jason Merrill <jason at redhat dot com>]

71913 is a case where unsafe_copy_elision_p was being too
conservative. We can allow copy elision in a new expression; the only
way we could end up initializing a base subobject without knowing it
would be through a placement new, in which case we would already be
using the wrong (complete object) constructor, so copy elision doesn't
make it any worse.

The second patch is for a case where we weren't being conservative
enough; we need to beware of tail padding even when the copy
constructor is trivial.

Tested x86_64-pc-linux-gnu, applying to trunk.  71913 patch also
applied to 6 branch.

Attachment: 71913.diff
Description: Text document

Attachment: trivial-elide.diff
Description: Text document