Add MachOLinkingContext parameter to MachOFileNode constructor.

No functionality change.  This removes a down-cast from LinkingContext to
MachOLinkingContext.

Also, remove const from LinkingContext::createImplicitFiles() to remove
the need for another const cast.  Seems reasonable for createImplicitFiles()
to need to modify the context (MachOLinkingContext does).

llvm-svn: 218796