[clang] [clang][PAC][darwin] Set correct default ptrauth features for arm64e-darwin (PR #153722)

[Oliver Hunt via cfe-commits]

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@@ -3291,6 +3291,39 @@ void Darwin::addClangTargetOptions(
     if (!RequiresSubdirectorySearch)
       CC1Args.push_back("-fno-modulemap-allow-subdirectory-search");
   }
+
+  if (getTriple().isArm64e()) {
+    auto EnsureDefaultPtrauthFlag = [&](OptSpecifier Pos, OptSpecifier Neg) {
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ojhunt wrote:

Does hasFlag(..) handle `-ffoo -fno-foo` correctly? I've assumed that it does not?

The reason for the weird push_back setup is because the verbose push_back("-ffoo") has always seemed error prone and difficult to maintain, and given these are a series of identically structured args it seemed doing the look up programmatically was better anyway?

https://github.com/llvm/llvm-project/pull/153722