[clang] [Clang] Mark this pointer in destructors dead_on_return (PR #166276)

[John McCall via cfe-commits]

rjmccall wrote:

> > I think it's worth doing this for non-virtual bases just to avoid needing an excessive number of controlling flags, since otherwise I assume people might want a staging flag that disables this just for classes with bases.
> 
> I've left this as a TODO, mainly so I can make sure that our internal release process doesn't catch many more issues than how I tested this. Then I plan on doing another round of testing without the check on non-virtual bases and then removing the TODO. We don't need more selective flags than this and I don't see why other users couldn't just be pointed at an opt-out flag. Is this strategy okay or would you like to see the non-virtual base check removed before landing?

I know there's been feedback before from Google folks about having a flag whenever we expand an optimization so that they can more easily check whether the expansion specifically caused a particular regression.

https://github.com/llvm/llvm-project/pull/166276