`with_taks_impl` is only called from `with_eval_always_task` and
`with_task` . The former is only used in query invocation, while the
latter is also used to start the `tcx` and to trigger codegen.

This move should not change significantly the number of calls to this
assertion.

try_execute_query is now able to centralize the path for query
get/ensure/force.

try_execute_query now takes the dep_node as a parameter, so it can
accommodate `force`. This dep_node is an Option to avoid computing it in
the `get` fast path.

try_execute_query now returns both the result and the dep_node_index to
allow the caller to handle the dep graph.

The caller is responsible for marking the dependency.

Remove with_eval_always_task.