[clang] [llvm] [PAC][Driver] Implement `-mbranch-protection=pauthabi` option (PR #97237)

[Peter Smith via llvm-commits]

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@@ -1537,11 +1570,16 @@ static void CollectARMPACBTIOptions(const ToolChain &TC, const ArgList &Args,
     if (!isAArch64 && PBP.Key == "b_key")
       D.Diag(diag::warn_unsupported_branch_protection)
           << "b-key" << A->getAsString(Args);
+    if (!isAArch64 && PBP.HasPauthABI)
+      D.Diag(diag::warn_unsupported_branch_protection)
+          << "pauthabi" << A->getAsString(Args);
     Scope = PBP.Scope;
     Key = PBP.Key;
     BranchProtectionPAuthLR = PBP.BranchProtectionPAuthLR;
     IndirectBranches = PBP.BranchTargetEnforcement;
     GuardedControlStack = PBP.GuardedControlStack;
+    if (isAArch64 && PBP.HasPauthABI)
----------------
smithp35 wrote:

Do we need any additional error checking for the existing branch protection options that affect pointer authentication?

For example we have
```
pac-ret // return address signing with A key
pac-ret+leaf // extend return address signing to leaf functions
standard = pac-ret+bti+pc // enable pac-ret, bti and pc (if available on hardware).
pc // Enable pc as modifier in return address signing.
b-key // Use b-key for signing return address.
```

When pauthabi is used, are the other PAC related options ignored? I can see `+leaf` being potentially useful, as well as `+pc`. I think b-key is going to clash with the signing schema.

The other options look to be subsets of pauthabi (unless additional command line options unless `-fno-ptrauth-returns` is used.

https://github.com/llvm/llvm-project/pull/97237