clang/Modules: Move Serialization/Module.{h,cpp} to ModuleFile, NFC

Remove some cognitive load by renaming clang/Serialization/Module.h to
clang/Serialization/ModuleFile.h, since it declares the ModuleFile
class.  This also makes editing a bit easier, since the basename of the
file no long conflicts with clang/Basic/Module.h, which declares the
Module class.  Also move lib/Serialization/Module.cpp to
lib/Serialization/ModuleFile.cpp.
diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
index 28affed..4c18662 100644
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -65,7 +65,7 @@
 #include "clang/Sema/Weak.h"
 #include "clang/Serialization/ASTReader.h"
 #include "clang/Serialization/InMemoryModuleCache.h"
-#include "clang/Serialization/Module.h"
+#include "clang/Serialization/ModuleFile.h"
 #include "clang/Serialization/ModuleFileExtension.h"
 #include "clang/Serialization/SerializationDiagnostic.h"
 #include "llvm/ADT/APFloat.h"