Replace _IRR_OVERRIDE_ macro with override keyword

Replace _IRR_OVERRIDE_ macro with override keyword

The commit also establishes a precedent of leaving off the `virtual`
keyword in overrides. Although not strictly necessary, I believe this is
good for readability because it makes it clear it is an override and not
a pure virtual function, and it helps keep line lengths shorter. We
should move towards eliminating the macro altogether, but the definition
has been left in with a note on deprecation so that in-progress work
will not suffer merge conflicts.