Rollup merge of #81176 - camsteffen:qpath-res, r=oli-obk

Improve safety of `LateContext::qpath_res`

This is my first rustc code change, inspired by hacking on clippy!

The first change is to clear cached `TypeckResults` from `LateContext` when visiting a nested item. I took a hint from [here](5e91c4ecc0/compiler/rustc_privacy/src/lib.rs (L1300)).

Clippy has a `qpath_res` util function to avoid a possible ICE in `LateContext::qpath_res`. But the docs of `LateContext::qpath_res` promise no ICE. So this updates the `LateContext` method to keep its promises, and removes the util function.

Related: rust-lang/rust-clippy#4545

CC ````````````@eddyb```````````` since you've done related work
CC ````````````@flip1995```````````` FYI

compiler/rustc_lint/src/context.rs

@@ -746,6 +746,14 @@ impl<'tcx> LateContext<'tcx> {
             hir::QPath::Resolved(_, ref path) => path.res,
             hir::QPath::TypeRelative(..) | hir::QPath::LangItem(..) => self
                 .maybe_typeck_results()
+                .filter(|typeck_results| typeck_results.hir_owner == id.owner)
+                .or_else(|| {
+                    if self.tcx.has_typeck_results(id.owner.to_def_id()) {
+                        Some(self.tcx.typeck(id.owner))
+                    } else {
+                        None
+                    }
+                })
                 .and_then(|typeck_results| typeck_results.type_dependent_def(id))
                 .map_or(Res::Err, |(kind, def_id)| Res::Def(kind, def_id)),
         }