This diff adds a `--disallow-inferred-unreachable` flag that
reports an error if:

1. Any branch is inferred to be unreachable
2. Any subexpression in boolean expressions, inline if statements,
   and list/set/generator/dict comprehensions are always unreachable
   or redundant.

This only takes into account the results of *type* analysis. We do not
report errors for branches that are inferred to be unreachable in the
semantic analysis phase (e.g. due to `sys.platform` checks and the
like).

Those checks are all intentional/deliberately flag certain branches
as unreachable, so error messages would be annoying in those cases.

A few additional notes:

1. I didn't spend a huge amount of time trying to come up with error
   messages. They could probably do with some polishing/rewording.

2. I thought about enabling this flag for default with mypy, but
   unfortunately that ended up producing ~40 to 50 (apparently
   legitimate) error messages.

   About a fourth of them were due to runtime checks checking to see
   if some type is None (despite not being declared to be Optional),
   about a fourth seemed to be due to mypy not quite understanding how
   to handle things like traits and Bogus[...], and about a fourth
   seemed to be legitimately unnecessary checks we could safely remove.

   The final checks were a mixture of typeshed bugs and misc errors with
   the reachability checks. (e.g. see python#7048)

3. For these reasons, I decided against adding this flag to `--strict`
   and against documenting it yet. We'll probably need to flush out
   a few bugs in mypy first/get mypy to the state where we can at least
   honestly dogfood this.

Resolves python#2395; partially
addresses python#7008.