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/context.cpp

@@ -712,17 +712,9 @@ class TypeCompleter {
     return value_rep;
   };
 
-  auto BuildImportRefUsedValueRepr(SemIR::TypeId type_id,
-                                   SemIR::ImportRefUsed import_ref) const
-      -> SemIR::ValueRepr {
-    CARBON_CHECK(import_ref.inst_id.is_builtin())
-        << "TODO: Handle non-builtin ImportRefUsed cases, such as functions, "
-           "classes, and interfaces";
-
-    const auto& import_ir = context_.import_irs().Get(import_ref.ir_id);
-    auto import_inst = import_ir->insts().Get(import_ref.inst_id);
-
-    switch (import_inst.As<SemIR::Builtin>().builtin_kind) {
+  auto BuildBuiltinValueRepr(SemIR::TypeId type_id,
+                             SemIR::Builtin builtin) const -> SemIR::ValueRepr {
+    switch (builtin.builtin_kind) {
       case SemIR::BuiltinKind::TypeType:
       case SemIR::BuiltinKind::Error:
       case SemIR::BuiltinKind::Invalid:
@@ -743,6 +735,16 @@ class TypeCompleter {
     llvm_unreachable("All builtin kinds were handled above");
   }
 
+  auto BuildImportRefUsedValueRepr(SemIR::TypeId type_id,
+                                   SemIR::ImportRefUsed import_ref) const
+      -> SemIR::ValueRepr {
+    const auto& import_ir = context_.import_irs().Get(import_ref.ir_id);
+    auto import_inst = import_ir->insts().Get(import_ref.inst_id);
+    CARBON_CHECK(import_inst.kind() != SemIR::InstKind::ImportRefUsed)
+        << "If ImportRefUsed can point at another, this would be recursive.";
+    return BuildValueRepr(type_id, import_inst);
+  }
+
   auto BuildStructOrTupleValueRepr(std::size_t num_elements,
                                    SemIR::TypeId elementwise_rep,
                                    bool same_as_object_rep) const
@@ -928,7 +930,7 @@ class TypeCompleter {
         return MakeEmptyValueRepr();
 
       case SemIR::Builtin::Kind:
-        CARBON_FATAL() << "Builtins should be named as ImportRefUsed";
+        return BuildBuiltinValueRepr(type_id, inst.As<SemIR::Builtin>());
 
       case SemIR::BindSymbolicName::Kind:
       case SemIR::PointerType::Kind: