Split deferred node traversal out from check.cpp (#4559)

I'm looking at adding more significant logic to checking, particularly
for interop. But check.cpp is getting large, and I think just adding
more logic will make it harder to reason about, so I'm looking at
splitting it up. This moves out NodeIdTraversal and
DeferredDefinitionWorklist because they're already independent from the
other code, and are reasonably sized to have their own files.

toolchain/check/BUILD

@@ -96,7 +96,11 @@ cc_library(
     name = "check",
     srcs = [
         "check.cpp",
+        "deferred_definition_worklist.cpp",
+        "deferred_definition_worklist.h",
         "handle.h",
+        "node_id_traversal.cpp",
+        "node_id_traversal.h",
     ] +
     # Glob handler files to avoid missing any.
     glob([