Note that the quick fix to implement header class works incorrectly when
that class has nested classes at the moment; this should be fixed
separately
#KT-15494 Fixed
#KT-18573 Fixed
Try to report most mismatch errors on the 'impl' declaration. Only
report a mismatch error on the 'header' declaration if no error would be
otherwise reported on any 'impl' declaration in the compilation unit.
Also render declaration kind in the message
#KT-18447 In Progress
