Another cross-cutting clang-tidy change. This time, we run the
clang-tidy modernize-use-override fixit over the entire code base.

The change, for virtual member functions in child classes, modifies this:

  virtual void Foo();

to

  void Foo() override;

and also strips any redundant "override final" (changed to only final),
"virtual ... override" (changed to only virtual) and the likes.

The primary advantage of using C++11 override is that it enables catching
subtle inheritance errors. In particular, if you change the prototype of a
member function in a base class and missed a derived class, the override
specifier will instantly tell you via a compiler error. Similarly, if you get
the types or number of parameters wrong in a derived class (and use
override), you will get a compiler error. And finally, if you create a member
function that happens to have the same signature as a base member class but
you didn't know about it (ie., don't use override), you will most likely get
an "inconsistent use of override" warning.

We already use override a fair amount in our case base (about 9000
instances), so this is mostly about getting into a consistent state without a
lot of manual labor.

Change-Id: I40802461dc97c7f97c1b688f4c9fc0ba3dba4794