[llvm] [AArch64][PAC] Protect the entire function if pac-ret+leaf is passed (PR #140895)

[Anatoly Trosinenko via llvm-commits]

atrosinenko wrote:

Ping.

I updated the test cases to better demonstrate the effect on function epilogue.

Furthermore, I returned to the original approach of interpreting the DWARF rules as 1) the CFA rule for `w30` defines the location of either signed or non-signed value of LR and 2) whether the value is signed is controlled by `RA_SIGN_STATE` pseudo register. It seems both approaches have drawbacks (surprising but seemingly correct interpretation of `.cfi_restore w30` vs. CFA rule pointing to potentially outdated value on the stack), but the original one adds less clutter to AArch64FrameLowering.

https://github.com/llvm/llvm-project/pull/140895