Make stack switching compatible with address sanitizer

[Keno]

Right now we set -mllvm -asan-stack=0 because address sanitizer does not know what to do with our stack switching. It would be good to fix that such that we can take advantage of this capability. My best guess as to how to fix it is that we'd have to also allocate a shadow map for each stack and then also save the shadow map and restore it when we switch back.

[vtjnash]

does msan let you do that? there's also #13099 which fixes the !COPY_STACKS build flag and never messes with the stack.

[Keno]

Yes, it does. You have to be a little bit careful, but you can ask it for a pointer to it's shadow memory and use an uninstrumented memcpy.