[wp] Warns on user defined variant relations

src/plugins/wp/cfgAnnot.ml

src/plugins/wp/cfgWP.ml

src/plugins/wp/tests/wp_acsl/oracle/decreases.0.res.oracle

src/plugins/wp/tests/wp_acsl/oracle/decreases.1.res.oracle

...ns/wp/tests/wp_acsl/oracle/terminates_formulae.res.oracle

...ins/wp/tests/wp_acsl/oracle_qualif/decreases.0.res.oracle

...ins/wp/tests/wp_acsl/oracle_qualif/decreases.1.res.oracle

...ests/wp_acsl/oracle_qualif/terminates_formulae.res.oracle