[clang] [clang][PP] Add extension to predefine target OS macros (PR #74676)

[Jon Roelofs via cfe-commits]

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@@ -1344,6 +1344,15 @@ static void InitializePredefinedMacros(const TargetInfo &TI,
   if (TI.getTriple().isOSBinFormatELF())
     Builder.defineMacro("__ELF__");
 
+  // Target OS macro definitions.
+  if (PPOpts.DefineTargetOSMacros) {
+    const llvm::Triple &Triple = TI.getTriple();
+#define TARGET_OS(Name, Predicate)                                             \
+  Builder.defineMacro(#Name, Predicate ? "1" : "0");
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jroelofs wrote:

`Predicate` should be in parens to make the precedence explicit in the face of preprocessor expansion.

https://github.com/llvm/llvm-project/pull/74676