[llvm] [clang] [AArch64] Make +pauth enabled in Armv8.3-a by default (PR #78027)

[David Spickett via cfe-commits]

https://github.com/DavidSpickett approved this pull request.

LGTM.

I think the reason we didn't do this is because we have so far:
* Used nop space encodings
* Used a CPU that would enable it
* Been adding -mbranch-protection and friends anyway, so adding +pauth didn't seem like a big deal.

But it's correct according to the ARMARM, so let's go ahead with this.

https://github.com/llvm/llvm-project/pull/78027