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

[Anatoly Trosinenko via llvm-commits]

atrosinenko wrote:

By the way, aside from protecting the value of LR when signal handler is called, signing LR immediately after entering the function seems to have another good side-effect: this eliminates an implicit BTI target at the `autiasp` instruction in the middle of the function.

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