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

[Stephen Lin]

> OK, sorry for the noise: it turns out the last patch I sent it doing
> the right thing, but largely be accident: it's not attaching
> 'returned' to the call site and only at the declaration/definition, so
> only direct calls end up using the 'returned' attribute, which is what
> we want.
>
> But this is actually because EmitCall treats the call as
> void-returning even when it returns 'this', because the return value
> is gets is based on semantic ReturnValueSlot rather than from the
> ABI-provided constructor signature, so ConstructAttributeList is
> called with a 'this'-return when constructing a declaration/definition
> but a 'void' return when constructing a call site. I'm surprised this
> doesn't trigger any asserts, actually...
>
> Anyway, since this is not behavior that should be relied upon (and
> maybe should be fixed at some point, too), I fixed it so that
> 'returned' is applied by SetFunctionAttributes(), which is only
> applied to declarations/definitions rather than call sites and takes a
> GlobalDecl already. So this is probably the minimal impact method of
> implementing this.
>
> Thanks for bringing up the thunk issue; would not have realized what
> was going on otherwise. Please let me know if you have any more
> suggestions or comments.
>
> -Stephen

Hi John,

No worries if not, but do you think you'll be able to review this in
time for the 3.3 branching on Monday?

If there's anything else I should refine about the implementation
please let me know.

-Stephen