Similarly to getFirstClassifierDiscriminateHeaders, we select the first
non-header class if possible, otherwise we select just the first class.
This makes sure that a reference will never be resolved to a header
class if the corresponding impl class is present.
Note that overall the issue may not be fixed yet, because there are
other scopes where header classes are not yet discriminated
(LazyImportScope, DeserializedMemberScope). However, at this point I
cannot reproduce this problem with these other scopes
#KT-15521 Fixed
