Calling `ClassDefSymbolAPI#method` may explode on polymorphic classes

[abgruszecki]

As in the title. This is a variant of #6515. We need to propagate the fix from https://github.com/lampepfl/dotty/pull/6521/files#diff-94f1cae70a9f01db6e3eecd271b7f759R1559 to all other places in the API where we call Type#allMembers (probably Type#decls as well?)