Add 'let' handling for ImportRefUsed. (#3700)

- File::StringifyTypeExpr now has a case that hits the ImportRefUsed
TODO, so implementing that. I think the `static` approach will be
helpful in ensuring there aren't access bugs, particularly when future
support is added.

- `let` wasn't adding to exports because it doesn't use
`decl_name_stack` the way `var` does. This now adds to exports, but we
might want to unify logic for issues such as this.

- BuildImportRefUsedValueRepr is now called for another inst kind, and
it seemed like calling back to BuildValueRepr was the best way to
resolve this. I don't *think* that's going to cause recursion.

toolchain/check/handle_let.cpp

@@ -71,6 +71,9 @@ auto HandleLetDecl(Context& context, Parse::LetDeclId parse_node) -> bool {
   // Add the name of the binding to the current scope.
   auto name_id = context.bind_names().Get(bind_name.bind_name_id).name_id;
   context.AddNameToLookup(name_id, pattern_id);
+  if (context.scope_stack().PeekNameScopeId() == SemIR::NameScopeId::Package) {
+    context.AddExport(pattern_id);
+  }
   return true;
 }