[clang] [llvm] [AArch64] Support preserve_none calling convention (PR #91046)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

X19 is the base register; can we actually allocate arguments in it in general?  This seems hard to fix.

It looks like frame lowering assumes X9 is available; that's probably fixable, but the code needs to be reworked, I think.

X15 is used on Windows for stack allocation; I think you can use it in this context, but probably worth a test to verify the interaction works the way you want.  (And more generally, that this calling convention doesn't explode on Windows targets.)

What's the interaction between varargs and preserve_none?

https://github.com/llvm/llvm-project/pull/91046