[SystemZ][z/OS] Add AutoConvert.h header to MemoryBuffer.cpp

This commit 75e164f removed the AutoConvert.h header causing a build break on z/OS. This patch adds it back to fix it.

Reviewed By: zibi

Differential Revision: https://reviews.llvm.org/D118129

llvm/lib/Support/MemoryBuffer.cpp

@@ -13,6 +13,7 @@
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/Config/config.h"
+#include "llvm/Support/AutoConvert.h"
 #include "llvm/Support/Error.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/Errc.h"