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These are a handful of patches fixing bugs with the current #[rustc_scalable_vector] infrastructure so that we can upstream the intrinsics to stdarch:

1. Instead of just using regular struct lowering for tuples of scalable vectors, which results in an incorrect ABI (e.g. returning indirectly), we change to using BackendRepr::ScalableVector which will lower to the correct type and be passed in registers.

2. Clang changed from representing tuples of scalable vectors as structs rather than as wide vectors (that is, scalable vector types where the N part of the <vscale x N x ty> type was multiplied by the number of vectors). rustc mirrored this in the initial implementation of scalable vectors that didn't land.

   When our early patches used the wide vector representation, our intrinsic patches used the legacy llvm.aarch64.sve.tuple.{create,get,set}{2,3,4} intrinsics for creating these tuples/getting/setting the vectors, which were only supported due to LLVM's AutoUpgrade pass converting these intrinsics into llvm.vector.insert. AutoUpgrade only supports these legacy intrinsics with the wide vector representation.

   With the current struct representation, Clang has special handling in codegen for generating insertvalue/extractvalue instructions for these operations, which must be replicated by rustc's codegen for our intrinsics to use.

   We add a new core::intrinsics::scalable module (never intended to be stable, just used by the stdarch intrinsics, gated by core_intrinsics) and add new intrinsics which rustc lowers to the appropriate insertvalue/extractvalue instructions.

3. Add generation of debuginfo for scalable vectors, following the DWARF that Clang generates.

4. Some intrinsics need something like simd_cast, which will work for scalable vectors too, so this implements a new sve_cast intrinsic that uses the same internals as simd_cast. This seemed better than permitting scalable vectors to be used with the simd_* intrinsics more generally as I can't guarantee this would work for all of them.

This is a relatively large patch but most of it is tests, and each commit should be relatively standalone. It's a little bit easier to upstream them together to avoid needing to stack them. It's possible that some more compiler fixes will be forthcoming but it's looking like this might be all at the moment.

Depends on #153653

r? @workingjubilee (discussed on Zulip)