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.