Revise dataflow to do a cfg-driven walk.

Fix #6298.

This is instead of the prior approach of emulating cfg traversal
privately by traversing AST in same way).

Of special note, this removes a special case handling of `ExprParen`
that was actually injecting a bug (since it was acting like an
expression like `(*func)()` was consuming `*func` *twice*: once from
`(*func)` and again from `*func`).  nikomatsakis was the first one to
point out that it might suffice to simply have the outer `ExprParen`
do the consumption of the contents (alone).

(This version has been updated to incorporate feedback from Niko's
review of PR 14873.)

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