This commit introduces `QPath::LangItem` to the HIR and uses it in AST
lowering instead of constructing a `hir::Path` from a slice of symbols.
This might be better for performance, but is also much cleaner as the
previous approach is fragile. In addition, it resolves a bug (#61019)
where an extern crate imported as "std" would result in the paths
created during AST lowering being resolved incorrectly (or not at all).
Co-authored-by: Matthew Jasper <mjjasper1@gmail.com>
Signed-off-by: David Wood <david@davidtw.co>
762137e212