[PATCH] Have 'this'-returning constructors and destructors to take advantage of the new backend 'returned' attribute

[Stephen Lin]

> CodeGenModule::ConstructAttributeList gets the Decl;  just have it
> check for a ctor or dtor and ask the CXXABI whether that Decl
> promises to return 'this'.

OK, that works...I didn't want to be the one that added a dependency
on CGCXXABI where none existed before, but since you're suggesting it
I'll go with it :)

-Stephen