Don't check equals, hashCode, toString in LoadBuiltinsTest

Otherwise there'll be too many fake overrides when these methods will appear in
Any
Check if the class is special by FQ name

This is needed because our built-ins can be loaded with two different
mechanisms (deserialization and lazy resolve) and, when that happens,
corresponding descriptors aren't equal to each other, so one of two resolved
descriptors for Any (or the same with Nothing) was never considered to be a
"special class" with no supertypes
Delete Hashable, pull up its members to Any

Extensions on nullable types remain in Library.kt for now, but will be moved to
stdlib later
Create fake overrides for deserialized enum class object

Now that Any has members, we need to make sure fake overrides are properly
generated in every class descriptor we create automatically or manually.

This fixes DescriptorSerializationTestGenerated
Minor, move method closer to its usage
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