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/lower/handle.cpp

@@ -200,6 +200,12 @@ auto HandleImport(FunctionContext& /*context*/, SemIR::InstId /*inst_id*/,
   FatalErrorIfEncountered(inst);
 }
 
+auto HandleImportRefUnused(FunctionContext& /*context*/,
+                           SemIR::InstId /*inst_id*/,
+                           SemIR::ImportRefUnused inst) -> void {
+  FatalErrorIfEncountered(inst);
+}
+
 auto HandleInitializeFrom(FunctionContext& context, SemIR::InstId /*inst_id*/,
                           SemIR::InitializeFrom inst) -> void {
   auto storage_type_id = context.sem_ir().insts().Get(inst.dest_id).type_id();
@@ -221,12 +227,6 @@ auto HandleIntLiteral(FunctionContext& context, SemIR::InstId inst_id,
   context.SetLocal(inst_id, v);
 }
 
-auto HandleLazyImportRef(FunctionContext& /*context*/,
-                         SemIR::InstId /*inst_id*/, SemIR::LazyImportRef inst)
-    -> void {
-  FatalErrorIfEncountered(inst);
-}
-
 auto HandleNameRef(FunctionContext& context, SemIR::InstId inst_id,
                    SemIR::NameRef inst) -> void {
   auto type_inst_id = context.sem_ir().types().GetInstId(inst.type_id);