Share the construction of the evar instance in Clenv.make_evar_clause.
Share the construction of the evar instance in Clenv.make_evar_clause.

Fixes most of #8822.

coq/coq,Zimmi48/coq,ppedrot/coq,ppedrot/coq,maximedenes/coq,gares/coq,JasonGross/coq,SkySkimmer/coq,Matafou/coq,maximedenes/coq,maximedenes/coq,ejgallego/coq,Matafou/coq,JasonGross/coq,JasonGross/coq,JasonGross/coq,gares/coq,gares/coq,SkySkimmer/coq,silene/coq,ppedrot/coq,Zimmi48/coq,gares/coq,ejgallego/coq,silene/coq,ejgallego/coq,ppedrot/coq,Matafou/coq,ejgallego/coq,coq/coq,SkySkimmer/coq,coq/coq,SkySkimmer/coq,maximedenes/coq,silene/coq,Zimmi48/coq,Zimmi48/coq,coq/coq,ppedrot/coq,maximedenes/coq,gares/coq