[clang][PAC] ptrauth_qualifier and ptrauth_intrinsic should only be available on Darwin (#153912)

For backwards compatibility reasons the `ptrauth_qualifier` and
`ptrauth_intrinsic` features need to be testable with `__has_feature()`
on Apple platforms, but for other platforms this backwards compatibility
issue does not exist.

This PR resolves these issues by making the `ptrauth_qualifier` and
`ptrauth_intrinsic` tests conditional upon a darwin target. This also
allows us to revert the ptrauth_qualifier check from an extension to a
feature test again, as is required on these platforms.

At the same time we introduce a new predefined macro `__PTRAUTH__` that
answers the same question as `__has_feature(ptrauth_qualifier)` and
`__has_feature(ptrauth_intrinsic)` as those tests are synonymous and
only exist separately for compatibility reasons.

The requirement to test for the `__PTRAUTH__` macro also resolves the
hazard presented by mixing the `ptrauth_qualifier` flag (that impacts
ABI and security policies) with `-pedantics-errors`, which makes
`__has_extension` return false for all extensions.

---------

Co-authored-by: Aaron Ballman <aaron@aaronballman.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/clang/lib/Frontend/InitPreprocessor.cpp b/clang/lib/Frontend/InitPreprocessor.cpp
index 5980806..4865c0b 100644
--- a/clang/lib/Frontend/InitPreprocessor.cpp
+++ b/clang/lib/Frontend/InitPreprocessor.cpp
@@ -1535,6 +1535,9 @@ static void InitializePredefinedMacros(const TargetInfo &TI,
 #undef TARGET_OS
   }
 
+  if (LangOpts.PointerAuthIntrinsics)
+    Builder.defineMacro("__PTRAUTH__");
+
   // Get other target #defines.
   TI.getTargetDefines(LangOpts, Builder);
 }