COMP: Prefer const member functions

Finds non-static member functions that can be made const because the functions
don’t use this in a non-const way.

This check tries to annotate methods according to logical constness (not
physical constness). Therefore, it will suggest to add a const qualifier to a
non-const method only if this method does something that is already possible
though the public interface on a const pointer to the object:

reading a public member variable calling a public const-qualified member
function returning const-qualified this passing const-qualified this as a
parameter.  This check will also suggest to add a const qualifier to a
non-const method if this method uses private data and functions in a limited
number of ways where logical constness and physical constness coincide:
 - reading a member variable of builtin type
Specifically, this check will not suggest to add a const to a non-const
method if the method reads a private member variable of pointer type because
that allows to modify the pointee which might not preserve logical constness.
For the same reason, it does not allow to call private member functions or
member functions on private member variables.

In addition, this check ignores functions that
 - are declared virtual
 - contain a const_cast
 - are templated or part of a class template
 - have an empty body
 - do not (implicitly) use this at all (see readability-convert-member-functions-to-static).