Misleading error message when returning 'this' by reference from class member

[akarpov89]

Version Used:
Master branch @ March 9
Steps to Reproduce:

Compile the following code:

class C {
    ref C M() => ref this;   
}

Expected Behavior:
The compiler emits error "Class members cannot return 'this' by reference".

Actual Behavior:
The compiler emits error "Struct members cannot return 'this' or other instance members by reference"

[akarpov89]

It is also possible to use message "Cannot return 'this' by reference" without specifying if the member belongs to class or struct.

[alrz]

Dup of #15704 and already has a fix here: #15706