[Serialization] Add missing include

forward decl is not sufficient for destroying a unique_ptr<MemoryBuffer>.

llvm-svn: 355816

clang/include/clang/Serialization/InMemoryModuleCache.h

@@ -12,12 +12,9 @@
 #include "llvm/ADT/IntrusiveRefCntPtr.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/StringMap.h"
+#include "llvm/Support/MemoryBuffer.h"
 #include <memory>
 
-namespace llvm {
-class MemoryBuffer;
-} // end namespace llvm
-
 namespace clang {
 
 /// In-memory cache for modules.