[CodeCompletion] Allow preChecking an expression twice in code completion

[ahoppen]

In code completion we might call preCheckExpression twice - once for the first pass and once for the second pass. This is fine since preCheckExpression idempotent, so don't assert if we are in code completion mode.

Fixes rdar://79136653 [SR-14755]

[ahoppen]

@swift-ci Please smoke test

[rintaro]

@xedin Should preCheckExpression() be idempotent (#33749 (comment))?

[xedin]

I think it should be yes, although there might be edge cases which cases these crashes...

[xedin]

Is this some kind of a stale pointer problem where sequence has been folded but something still holds a reference to an original version?

[rintaro]

@xedin Yeah, some expression kinds e.g. AssignExpr in SequenceExpr are mutated in precheckExpression()

[rintaro]

Maybe we can just disable these assertions (!expr->isFolded()) when we are in code completion mode?

[xedin]

I think it would be okay. Real solution here would be to type-check multi-statement closures together with their context just like single-expression ones, once that's done we should be able to remove all this complicated fallback logic.

[ahoppen]

OK, that’s the even simpler fix. I disabled the asserts if CompletionCallback is set.

[ahoppen]

@swift-ci Please smoke test