Fix dataclass plugin to work with new semantic analyzer

[ilevkivskyi]

Fixes #6324

This enables two test files that previously had few crashes.

Changes in this PR:

• Add defer() to public plugin API, otherwise dataclasses will not works with forward references.
• Update one plugin test to use the new analyzer because the output is a bit more verbose, and probably not worth the hassle of making it identical to old analyzer.
• One test is skipped because of existing issue New semantic analyzer: should we collect functions from AST instead of symbol tables? #6454
• Add the generated type variable used for self types to the class body (this is identical to named tuples and attrs plugin).

[JukkaL]

LGTM