[Sema] Fold SequenceExpr in pre-checking pre-walk

[hamishknight]

Doing it in the post-walk meant we ended up walking the children twice, which led to duplicate diagnostics and incorrect inference of the level of application for function references. Move it to the pre-walk, ensuring that we resolve any operator references before folding.

Resolves #75527

[hamishknight]

@swift-ci please test

[hamishknight]

@swift-ci please test source compatibility

[hamishknight]

@swift-ci please smoke test