CachePruning.h - reduce StringRef.h to Optional.h include. NFC

We only need to include Optional.h, forward declare StringRef and move the StringRef.h include down to CachePruning.cpp.

llvm/include/llvm/Support/CachePruning.h

@@ -14,12 +14,13 @@
 #ifndef LLVM_SUPPORT_CACHE_PRUNING_H
 #define LLVM_SUPPORT_CACHE_PRUNING_H
 
-#include "llvm/ADT/StringRef.h"
+#include "llvm/ADT/Optional.h"
 #include <chrono>
 
 namespace llvm {
 
 template <typename T> class Expected;
+class StringRef;
 
 /// Policy for the pruneCache() function. A default constructed
 /// CachePruningPolicy provides a reasonable default policy.

llvm/lib/Support/CachePruning.cpp

@@ -11,7 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Support/CachePruning.h"
-
+#include "llvm/ADT/StringRef.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/Errc.h"
 #include "llvm/Support/Error.h"