[Downstream Change] PAC-RET hardening against the PACMAN attack

[vhscampos]

What is the change
Command-line option, function attribute, and codegen for the PACMAN mitigation using a load of the return address.

Their naming is still up for debate in the upstream review, therefore it can change in future versions of the toolchain.

Why this change cannot be done upstream
It is being done upstream, but it is still in review. We need the changes for the ATFE 22 release, hence the downstream patch.

List of upstream PRs:

• [AArch64] Mark X16 as clobbered in PAUTH_EPILOGUE for hint-based PAuthLR llvm/llvm-project#175991
• [Clang][AArch64] Command-line options for A-profile's Sign Return Address Hardening llvm/llvm-project#176171
• [AArch64] Codegen for AArch64 Return Address Signing Hardening llvm/llvm-project#176187