Add coding standard recommending use of qualifiers in cpp files

There is prior art for this in the code base itself, and a recent
example of this here: c45f8d49

This came up in discussion on this review where @maskray was going the
opposite direction:
  https://reviews.llvm.org/D68772

Given that there is disagreement, we should make a choice and document
it.

Thanks to John McCall for the precise wording.

Reviewed By: MaskRay, rjmccall

Differential Revision: https://reviews.llvm.org/D74515