[llvm] [PAC][IR][AArch64] Add "ptrauth(...)" Constant to represent signed pointers. (PR #85738)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

I'm a little concerned that various places which check for `isa<ConstantExpr>` won't interact correctly with this: prior to this patch, the only possible constant expressions were ConstantData, GlobalValue, ConstantExpr, and ConstantAggregate.  This introduces another possibility... and I'm not sure everything is prepared for it.  Some stuff falls back gracefully, but I'm not sure everything does (e.g. Constant::containsConstantExpression).

Fixing that assumption is something we want to do eventually; so not really a problem, but consider spending a bit of time to try to find issues before we trip over them on user code.

https://github.com/llvm/llvm-project/pull/85738