[wp] Fixes opaque structures handling

src/plugins/wp/CodeSemantics.ml

src/plugins/wp/Cvalues.ml

src/plugins/wp/Definitions.ml

src/plugins/wp/Definitions.mli

src/plugins/wp/LogicSemantics.ml

src/plugins/wp/MemLoader.ml

src/plugins/wp/MemLoader.mli

src/plugins/wp/MemMemory.ml

src/plugins/wp/MemMemory.mli

src/plugins/wp/MemRegion.ml

src/plugins/wp/MemTyped.ml

src/plugins/wp/MemVar.ml

src/plugins/wp/ProverCoq.ml

src/plugins/wp/ProverErgo.ml

src/plugins/wp/ProverWhy3.ml

src/plugins/wp/tests/wp_acsl/opaque_struct.i

src/plugins/wp/tests/wp_acsl/oracle/opaque_struct.res.oracle

...s/wp/tests/wp_acsl/oracle_qualif/opaque_struct.res.oracle