Miscellaneous const-related fixes

[camelid]

• Remove parent def lookup hack that is no longer needed
• Fix lowering of CStr constants to valtrees

[rustbot]

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[rust-log-analyzer]

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##[endgroup]
[TIMING:end] tool::ToolBuild { build_compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu, tool: "tidy", path: "src/tools/tidy", mode: ToolBootstrap, source_type: InTree, extra_features: [], allow_features: "", cargo_args: [], artifact_kind: Binary } -- 11.041
[TIMING:end] tool::Tidy { compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu } -- 0.000
fmt check
Diff in /checkout/compiler/rustc_mir_build/src/thir/constant.rs:58:
         (ast::LitKind::Byte(n), ty::Uint(ty::UintTy::U8)) => {
             ty::ValTree::from_scalar_int(tcx, n.into())
         }
-        (ast::LitKind::CStr(byte_sym, _), ty::Ref(_, inner_ty, _)) if matches!(inner_ty.kind(), ty::Adt(def, _) if tcx.is_lang_item(def.did(), LangItem::CStr)) => {
+        (ast::LitKind::CStr(byte_sym, _), ty::Ref(_, inner_ty, _)) if matches!(inner_ty.kind(), ty::Adt(def, _) if tcx.is_lang_item(def.did(), LangItem::CStr)) =>
+        {
             // A CStr is a newtype around a byte slice, so we create the inner slice here.
             // We need a branch for each "level" of the data structure.
             let bytes = ty::ValTree::from_raw_bytes(tcx, byte_sym.as_byte_str());
fmt: checked 6473 files
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Build completed unsuccessfully in 0:00:47
  local time: Mon Oct 13 15:54:27 UTC 2025
  network time: Mon, 13 Oct 2025 15:54:27 GMT