[clang] [llvm] [clang][mlibc] Predefine `__mlibc__` for mlibc targets (PR #225654)

[Eli Friedman via llvm-commits]

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@@ -393,7 +393,9 @@ class LLVM_LIBRARY_VISIBILITY LinuxTargetInfo : public OSTargetInfo<Target> {
         // defined for compatibility.
         Builder.defineMacro("__ANDROID_API__", "__ANDROID_MIN_SDK_VERSION__");
       }
-    } else {
+    } else if (Triple.isMlibc()) {
+        Builder.defineMacro("__mlibc__");
+    } else if (Triple.isGNUEnvironment()) {
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efriedma-quic wrote:

Please leave out the isGNUEnvironment() check in the initial patch, to keep this about mlibc specifically, so this doesn't have unexpected effects.  Maybe we can do this in a followup.

https://github.com/llvm/llvm-project/pull/225654