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

[Eli Friedman via llvm-commits]

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@@ -848,7 +848,8 @@ class Triple {
   bool isOSGlibc() const {
     return (getOS() == Triple::Linux || getOS() == Triple::KFreeBSD ||
             getOS() == Triple::Hurd) &&
-           !isAndroid() && !isMusl() && getEnvironment() != Triple::PAuthTest;
+           !isAndroid() && !isMusl() && !isMlibc() &&
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efriedma-quic wrote:

This change is probably what we want, but I'd prefer a separate patch, and maybe include appropriate testcases.  (We check isOSGlibc() in a bunch of places in the backend, and some of them might need to be adjusted for mlibc.)

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