Fixes #21137

This flag has nothing to with type checking strictness, it is a
convenience option for config maintenance.

Fixes #21134

Helps with confusing symptoms in #21132

It is unfortunate that it makes the logic a little more ad hoc / might
lead to minimal repros being a little more confusing. But I think this
is still a better and more helpful state to be in than before
#20660 (relevant PR from the
narrowing rewrite).

Hopefully we can add the ability to check unreachable code, which will
fix this and other issues.

Fixes #21149 (the regression part of it, still working on the bad
behaviour present in previous mypy versions)

Fixes #21143 (on the same lines as #20704, i.e. there still exists a
deeper fix that could be made)

I also update a comment that is a little out of date since #20863 and
code in checkexpr.py that is out of date for modern typeshed

Fixes #21152

Similar to #20662

Fixes #21173

Fixes #21189

Fixes #21141

I am not sure how typeshed patches work. If possible, I would add some
tests here, otherwise I will add tests in a follow-up PR.

This is to unbreak master. We can apply an actual fix later.

Refs for context:
* #21120
*
python/cpython@e31c551

cc @JukkaL

Fixes #21176

This seems to resolve all the HDD performance issues. Although this
makes cache less durable, this is fine, as FS is not perfect in this
sense either (and it doesn't really need to be that durable).