objlib: Allow OBJECT libraries to link to OBJECT libraries.

Note: This still does not link the object-files of the `OBJECT` library
from the right-hand side of `target_link_libraries` to the target on the
left-hand side. (In this particular case of another `OBJECT` library on
the left-hand side this would not make any sense anyway. The target on
the left-hand side has no link-step.)

Issue: #14778

Source/cmGeneratorTarget.cxx

@@ -1054,9 +1054,6 @@ void cmGeneratorTarget::ComputeKindedSources(KindedSources& files,
       kind = SourceKindHeader;
     } else if (sf->GetPropertyAsBool("EXTERNAL_OBJECT")) {
       kind = SourceKindExternalObject;
-      if (this->GetType() == cmStateEnums::OBJECT_LIBRARY) {
-        badObjLib.push_back(sf);
-      }
     } else if (!sf->GetLanguage().empty()) {
       kind = SourceKindObjectSource;
     } else if (ext == "def") {