Auto merge of #122048 - erikdesjardins:inbounds, r=oli-obk

Use GEP inbounds for ZST and DST field offsets

ZST field offsets have been non-`inbounds` since I made [this old layout change](https://github.com/rust-lang/rust/pull/73453/files#diff-160634de1c336f2cf325ff95b312777326f1ab29fec9b9b21d5ee9aae215ecf5). Before that, they would have been `inbounds` due to using `struct_gep`. Using `inbounds` for ZSTs likely doesn't matter for performance, but I'd like to remove the special case.

DST field offsets have been non-`inbounds` since the alignment-aware DST field offset computation was first [implemented](erikdesjardins/rust@a2557d4#diff-04fd352da30ca186fe0bb71cc81a503d1eb8a02ca17a3769e1b95981cd20964aR1188) in 1.6 (back then `GEPi()` would be used for `inbounds`), but I don't think there was any reason for it.

Split out from #121577 / #121665.

r? `@oli-obk`

cc `@RalfJung` -- is there some weird situation where field offsets can't be `inbounds`?

Note that it's fine for `inbounds` offsets to be one-past-the-end, so it's okay even if there's a ZST as the last field in the layout:

> The base pointer has an in bounds address of an allocated object, which means that it points into an allocated object, or to its end. [(link)](https://llvm.org/docs/LangRef.html#getelementptr-instruction)

For rust-lang/unsafe-code-guidelines#93, zero-offset GEP is (now) always `inbounds`:

> Note that getelementptr with all-zero indices is always considered to be inbounds, even if the base pointer does not point to an allocated object. [(link)](https://llvm.org/docs/LangRef.html#getelementptr-instruction)