Basic check support for adapters. (#3874)

No support is provided yet for converting between a type and an adapter
for that type.

toolchain/lower/handle.cpp

@@ -26,6 +26,11 @@ static auto FatalErrorIfEncountered(InstT inst) -> void {
       << inst;
 }
 
+auto HandleAdaptDecl(FunctionContext& /*context*/, SemIR::InstId /*inst_id*/,
+                     SemIR::AdaptDecl inst) -> void {
+  FatalErrorIfEncountered(inst);
+}
+
 auto HandleAddrOf(FunctionContext& context, SemIR::InstId inst_id,
                   SemIR::AddrOf inst) -> void {
   context.SetLocal(inst_id, context.GetValue(inst.lvalue_id));