Fix #11827 FP duplInheritedMember - different return type

[chrchr-github]

No description provided.

[chrchr-github]

It seems that the functionConst examples "worked" only by accident.

Edit: There was an issue in CheckClass. Still not sure if we should warn for the examples from https://trac.cppcheck.net/ticket/11499
There is no guarantee that const/non-const overloads are interchangeable.
We don't warn for this equivalent code either:

struct A {
	void f() const;
};
template<class T>
struct P {
	T* get();
	const T* get() const;
};
struct S {
	P<A> p;
	void g() { p.get()->f(); }
};