Auto merge of #47009 - eddyb:issue-46855, r=arielb1

rustc_trans: support ZST indexing involving uninhabited types.

Fixes #46855 in a minimal way. I decided against supporting non-memory `Rvalue::Len` in this PR (see https://github.com/rust-lang/rust/issues/46855#issuecomment-352965807), as `PlaceContext::Inspect` is also used for `Rvalue::Discriminant`.

r? @arielb1