[llvm] [VFABI] Add support for vector functions that return struct types (PR #119000)

[David Sherwood via llvm-commits]

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david-arm wrote:

Is it worth having a negative test for an unpacked or literal struct, or perhaps for a struct of structs or a struct with arrays? For the latter two cases, I'd expect `getScalableECFromSignature` to fail sensibly.

https://github.com/llvm/llvm-project/pull/119000