[C++11] Add 'override' keywords and remove 'virtual'. Additionally add 'final' and leave 'virtual' on some methods that are marked virtual without overriding anything and have no obvious overrides themselves. XCore edition

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207501 91177308-0d34-0410-b5e6-96231b3b80d8