Rewrite `StackNesting` to use a single-pass algorithm#84688

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rjmccall wants to merge 2 commits intoswiftlang:mainfrom

rjmccall:stack-nesting-2

Commits on Oct 7, 2025

Document SIL's dominance rules, which apparently we've never done.
rjmccall
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Rewrite StackNesting to be a non-iterative single-pass algorithm.
rjmccall
committed