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).

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@218796 91177308-0d34-0410-b5e6-96231b3b80d8