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

[John McCall via cfe-commits]

rjmccall wrote:

> Given you said this is in general, it sounds like there are cases where virtual bases are not live out of the base subobject destructor?

Just sloppy wording, sorry. Virtual bases are always live out of the base subobject destructor.

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