Changes how unreachable statements are detected, refs #11437

[sobolevn]

Closes #11437

I suspect that some mypy / mypy_primer tests can fail after this change.
But, it looks logical and consistent to me.

[JukkaL]

The overall idea seems good, but I think that there may be various tweaks that could make this even better.

[JukkaL]

I'm wondering if this should be treated as unreachable code as well. Maybe the empty statement check for the following statements should be stricter, so that only pass, ... and maybe reveal_type(...) would be considered as empty, and maybe asserts.

[sobolevn]

I think that the initial motivation was to allow some "debug" code to reduce the number of false positives. For example, I can see some cases where people would use code like:

• assert False, 'unreachable'
• raise RuntimeError('should not be reached')
• sys.exit(1) # should not be reached

[sobolevn]

Thanks a lot, @JukkaL! I've addressed all your comments, except this one

Now:

• reveal_type and reveal_locals are always considered "reachable"
• Module level reachability has better locations

[KotlinIsland]

@sobolevn looks like #10916 is very similar/duplicate of #11437, do you think this pr will resolve it as well?