[clang] [Clang] __has_builtin should return false for aux triple builtins (PR #121839)

[Aiden Grossman via cfe-commits]

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@@ -1818,8 +1819,21 @@ void Preprocessor::ExpandBuiltinMacro(Token &Tok) {
             // usual allocation and deallocation functions. Required by libc++
             return 201802;
           default:
+            // We may get here because of aux builtins which may not be
+            // supported on the default target, for example if we have an X86
+            // specific builtin and the current target is SPIR-V. Sometimes we
+            // rely on __has_builtin returning true when passed a builtin that
+            // is not supported on the default target due to LangOpts but is
+            // supported on the aux target. See
+            // test/Headers/__cpuidex_conflict.c for an example. If the builtin
+            // is an aux builtin and it can never be supported on the default
+            // target, __has_builtin should return false.
+            if (getBuiltinInfo().isAuxBuiltinID(BuiltinID) &&
+                getBuiltinInfo().isAuxBuiltinIDAlwaysUnsupportedOnDefaultTarget(
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boomanaiden154 wrote:

I remember it being difficult/undesirable to gate this on the presence of `-fms-extensions`. I had a patch up that did that, and I remember @rnk commenting that wasn't the right approach. I need to go dig the stuff up and my bandwidth is a bit limited right now.

https://github.com/llvm/llvm-project/pull/121839