[llvm] [SystemZ][z/OS] Refactor AutoConvert.h to remove large MVS guard (PR #143174)

[Sean Perry via llvm-commits]

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@@ -894,21 +904,23 @@ void raw_fd_ostream::anchor() {}
 raw_fd_ostream &llvm::outs() {
   // Set buffer settings to model stdout behavior.
   std::error_code EC;
-#ifdef __MVS__
-  EC = enablezOSAutoConversion(STDOUT_FILENO);
+
+  // On z/OS we need to enable auto conversion
+  EC = enableAutoConversion(STDOUT_FILENO);
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perry-ca wrote:

pre-existing but the `enableAutoConversion()` should be only run on the first call to this function.  Same for errs.

https://github.com/llvm/llvm-project/pull/143174