[clang] [Clang][AArch64] Fix Pure Scalables Types argument passing and return (PR #112747)

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic commented:

Clang tracks the number of registers used on other targets, so it's not unprecedented... just an unfortunate consequence of the limitations of ABI in LLVM IR.

https://github.com/llvm/llvm-project/pull/112747