Add back include for AutoConvert.h as it's needed for z/OS (llvm#135430)

The commit
llvm@a1935fd
removed an include that is needed when building on z/OS.

Author: perry-ca

clang/lib/Basic/SourceManager.cpp

@@ -24,6 +24,7 @@
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/StringSwitch.h"
 #include "llvm/Support/Allocator.h"
+#include "llvm/Support/AutoConvert.h"
 #include "llvm/Support/Capacity.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/Endian.h"