Purge unused includes throughout libSupport.

NFC.

llvm-svn: 232976
diff --git a/llvm/lib/Support/Program.cpp b/llvm/lib/Support/Program.cpp
index b84b82b..34e336b 100644
--- a/llvm/lib/Support/Program.cpp
+++ b/llvm/lib/Support/Program.cpp
@@ -12,6 +12,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Support/Program.h"
+#include "llvm/ADT/StringRef.h"
 #include "llvm/Config/config.h"
 #include <system_error>
 using namespace llvm;