Refactor LazyImportRef into ImportRefUnused. (#3656)

This makes some changes to the formatter so that ImportRefUnused and
ImportRefUsed will both be labeled as "import_ref" with an "unused ->
used" argument change in textual IR, but is otherwise not changing
logic.

toolchain/check/import.cpp

@@ -104,7 +104,7 @@ static auto CopySingleNameScopeFromImportIR(
 
   // Produce the namespace for the entry.
   auto ref_id = context.AddInst(
-      SemIR::LazyImportRef{.ir_id = ir_id, .inst_id = import_inst_id});
+      SemIR::ImportRefUnused{.ir_id = ir_id, .inst_id = import_inst_id});
   auto namespace_inst =
       SemIR::Namespace{namespace_type_id, SemIR::NameScopeId::Invalid, ref_id};
   // Use the invalid node because there's no node to associate with.
@@ -211,7 +211,7 @@ auto Import(Context& context, SemIR::TypeId namespace_type_id,
     } else {
       // Leave a placeholder that the inst comes from the other IR.
       auto target_id = context.AddPlaceholderInst(
-          {SemIR::LazyImportRef{.ir_id = ir_id, .inst_id = import_inst_id}});
+          {SemIR::ImportRefUnused{.ir_id = ir_id, .inst_id = import_inst_id}});
       // TODO: When importing from other packages, the scope's names should
       // be changed to allow for ambiguous names. When importing from the
       // current package, as is currently being done, we should issue a