Rollup merge of #161729 - RalfJung:miri, r=RalfJung

miri subtree update

I want to add proper vararg checks for Miri's shims but it turns out I'll have to adjust the rustc side for this. It'll be easier to just do the entire thing as a rustc patch, but that needs a sync first.

---

Subtree update of `miri` to https://github.com/rust-lang/miri/commit/40000c5a938d61e91a410d6536d661b1ce6b0d35.

Created using https://github.com/rust-lang/josh-sync.

r? @ghost