C.128 override, virtual keyword handling

C.128 override, virtual keyword handling

Summary:
According to [C128] "Virtual functions should specify exactly one
of `virtual`, `override`, or `final`", I've added override where a
virtual function is overriden but the explicit `override` keyword
was missing. Whenever both `virtual` and `override` were specified,
I removed `virtual`. As C.128 puts it:

> [...] writing more than one of these three is both redundant and
> a potential source of errors.

I anticipate a discussion about whether or not to add `override` to
destructors but I went for it because of an example in [ISOCPP1000].
Let me repeat the comment for you here:

Consider this code:

```
    struct Base {
      virtual ~Base(){}
    };

    struct SubClass : Base {
      ~SubClass() {
        std::cout << "It works!\n";
      }
    };

    int main() {
      std::unique_ptr<Base> ptr = std::make_unique<SubClass>();
    }
```

If for some odd reason somebody removes the `virtual` keyword from the
`Base` struct, the code will no longer print `It works!`. So adding
`override` to destructors actively protects us from accidentally
breaking our code at runtime.

[C128]: https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#c128-virtual-functions-should-specify-exactly-one-of-virtual-override-or-final
[ISOCPP1000]: https://github.com/isocpp/CppCoreGuidelines/issues/1000#issuecomment-476951555

Reviewers: teemperor, JDevlieghere, davide, shafik

Reviewed By: teemperor

Subscribers: kwk, arphaman, kadircet, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D61440

llvm-svn: 359868