[C++11] Add 'override' keyword to virtual methods that override their base class.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@203769 91177308-0d34-0410-b5e6-96231b3b80d8
Tests for DR389-399.


git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@203771 91177308-0d34-0410-b5e6-96231b3b80d8