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.)
75340f4176