give symbol table quantified variables same unique ID as originator

In this commit we force the synthetic VarDecl in the symbol table that
corresponds to a quantified variable to have the same unique identifier as the
originating quantifier. Taking a global perspective, I think this is probably a
bad idea. This induces multiple differently typed AST nodes with the same unique
identifier. Everything matches up conveniently in the SMT bridge, but only sort
of coincidentally. A better approach may be to take a step back and construct a
VarDecl in a quantifier in the first place.