resolve: Preserve reexport chains in ModChildren

This may be potentially useful for
- avoiding uses of `hir::ItemKind::Use`
- preserving documentation comments on all reexports
- preserving and checking stability/deprecation info on reexports
- all kinds of diagnostics

compiler/rustc_middle/src/query/erase.rs

@@ -235,7 +235,6 @@ trivial! {
     rustc_hir::OwnerId,
     rustc_hir::Upvar,
     rustc_index::bit_set::FiniteBitSet<u32>,
-    rustc_middle::metadata::ModChild,
     rustc_middle::middle::dependency_format::Linkage,
     rustc_middle::middle::exported_symbols::SymbolExportInfo,
     rustc_middle::middle::resolve_bound_vars::ObjectLifetimeDefault,