[NFC] Use SubtypingDiscoverer in StringLowering #6325
Merged
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This replaces horrible hacks to find which nulls need to switch (from none to
noext) with general code using
SubtypingDiscoverer
. That helper is aware ofwhere each expression is written, so we can find those nulls trivially.
This is NFC on existing usage but should fix any remaining bugs in that area.
It is also shorter code.