This will mean users opting to not activate `cfg(test)` will lose IDE experience on them, which is quite unfortunate, but this is unavoidable if we want to avoid false positives on e.g. diagnostics. The real fix is to provide IDE experience even for cfg'ed out code, but this is out of scope for this PR.
Previously some expansions were not cached, but were cached in the expansion cache, which caused panics when later queries tried to lookup the node from the expansion cache.
This is a small change, but it was the cause of 90% of the errors in `rust-analyzer diagnostics .` 🫢
With this change and #18085 together, all remaining errors are type errors.
This may mean we can enable more errors, but this is out of scope for this PR.
