Don't use joins during semantics analysis for typed dict

[JukkaL]

See discussion at #2342.

[JukkaL]

Some ideas:

1. Leave the fallback empty early during semantic analysis and fill it in as the final pass of semantic analysis.
2. Make the fallback a property and calculate it lazily during type checking.

We probably don't want another full AST traversal just for this, as it would be expensive.