Do not use types as error contexts during type checking

[ilevkivskyi]

Fixes #16175
Fixes #8592

Situations where an error has a wrong line number are very annoying (maybe even more than a crash). Using types as error context during type checking is really risky. So, I removed all such cases in checker.py in checkexpr.py (by locally removing Context as a base class for Type). Most remaining uses are in semanal.py and typeanal.py. Those are less risky, and trickier to remove.

Notes:

• This will cause less precise error locations, but IMO it is much better than some random bizarre locations.
• I only add tests for situations with known repros.
• While I am at it, I do not emit variance errors for lambdas, unless I a missing something, those are bogus.

cc @JukkaL

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅