[ELF] Add "override" and remove "virtual".

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205056 91177308-0d34-0410-b5e6-96231b3b80d8
[MachO] Remove "virtual" and add "override".

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205057 91177308-0d34-0410-b5e6-96231b3b80d8