Function rustc_ast_lowering::item::add_ty_alias_where_clause

fn add_ty_alias_where_clause(
    generics: &mut Generics,
    where_clauses: (TyAliasWhereClause, TyAliasWhereClause),
    prefer_first: bool
)

When we have a ty alias we may have two where clauses. To give the best diagnostics, we set the span to the where clause that is preferred, if it exists. Otherwise, it sets the span to the other where clause if it exists.