Language changes for paper

These changes simplify the Dagrt language and the code builder
interface.

The first change is that *depends_on* becomes a computed attribute of
the phase DAG. The semantics of the DAG is changed so that *all*
statements get executed, instead of just those reachable from
*depends_on*.  This is a non-backwards compatible change that means
that DAGs are no longer allowed to carry unreachable "dead" code.

The language version is bumped to 2019.3.

The code builder interface is changed to remove the function
*reset_dep_tracking* and the attribute *phase_dependencies*.  Code
that is generated by the code builder should still function as
previously. The dependency DAG that is generated by the code builder
no longer uses Nops as placeholders, which simplifies its output.

cc: @cmikida, @inducer