Special handling of Ltac2 thunks.
Special handling of Ltac2 thunks.

Instead of introducing a dummy case node, we use an anonymous variable
annotated with the unit type.

Fixes #15616.

Zimmi48/coq,coq/coq,coq/coq,ejgallego/coq,ejgallego/coq,Zimmi48/coq,coq/coq,SkySkimmer/coq,SkySkimmer/coq,SkySkimmer/coq,coq/coq,Zimmi48/coq,SkySkimmer/coq,ejgallego/coq,Zimmi48/coq,ejgallego/coq