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

[Sean Perry via llvm-commits]

================
@@ -25,6 +26,16 @@
 #define CCSID_UTF_8 1208
 #define CCSID_ISO8859_1 819
 
+#ifndef STDIN_FILENO
+#define STDIN_FILENO 0
+#endif
+#ifndef STDOUT_FILENO
+#define STDOUT_FILENO 1
+#endif
+#ifndef STDERR_FILENO
+#define STDERR_FILENO 2
+#endif
----------------
perry-ca wrote:

I'd suggest moving these to InitLLVM.cpp & raw_ostream.cpp.  They should bechecked/ defined after unistd.h is included or we could get into a timing problem defining them here.  

Another option is to change the code to use `fileno(stderr)` instead of `STDERR_FILENO`.

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