Fix type comments crash inside generic definitions (#16849)
Closes https://github.com/python/mypy/issues/16649
It's the first time I am contributing to mypy so I am not very familiar
with how it works entirely behind the scene. The issue that I had is
that a crash happens when using tuple type comments inside
functions/classes that depend on a *constrained* type variable.
After investigation, the reason is that the type checker generates all
possible definitions (since constraints are known) and expands the
functions definitions and bodies accordingly. However, by doing so, a
tuple type comment ('# type: (int, float)') would have a FakeInfo, so
`ExpandTypeVisitor` would fail since it queries `t.type.fullname`.
By the way, feel free to change where my test should lie.
