Rollup merge of #117095 - klinvill:smir-fn-arg-count, r=oli-obk

Add way to differentiate argument locals from other locals in Stable MIR

This PR resolves rust-lang/project-stable-mir#47 which request a way to differentiate argument locals in a SMIR `Body` from other locals.

Specifically, this PR exposes the `arg_count` field from the MIR `Body`. However, I'm opening this as a draft PR because I think there are a few outstanding questions on how this information should be exposed and described. Namely:

- Is exposing `arg_count` the best way to surface this information to SMIR users? Would it be better to leave `arg_count` as a private field and add public methods (e.g. `fn arguments(&self) -> Iter<'_, LocalDecls>`) that may use the underlying `arg_count` info from the MIR body, but expose this information to users in a more convenient form? Or is it best to stick close to the current MIR convention?
- If the answer to the above point is to stick with the current MIR convention (`arg_count`), is it reasonable to also commit to sticking to the current MIR convention that the first local is always the return local, while the next `arg_count` locals are always the (in-order) argument locals?
- Should `Body` in SMIR only represent function bodies (as implied by the comment I added)? That seems to be the current case in MIR, but should this restriction always be the case for SMIR?

r? `@celinval`
r? `@oli-obk`

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