ovsdb-idlc: Simplify code generation to parse sets and maps of references.

ovsdb-idlc: Simplify code generation to parse sets and maps of references.

This switches from code that looks like:
    if (keyRow) {
        ...
    }
to:
    if (!keyRow) {
        continue;
    }
    ...
which is a little easier to generate because the indentation of ... is
constant.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Andy Zhou <azhou@ovn.org>