Merge AArch64Sandbox and CheriBoundAllocas
It would be good to use the target-independent stack allocation bounds pass to reduce differences between Morello and CHERI-RISC-V. Will need to investigate if there are any remaining optimizations in AArch64Sandbox that aren't handled by CheriBoundAllocas.
I noticed this because one of the CHERI-Generic tests was failing due to alloca names being lost inside AArch64Sandbox.cpp.
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