Fix new style union aliases with not ready items

[ilevkivskyi]

Fixes #21263
Fixes #11887

Old style aliases with new style unions are quite tricky, since this requires some heuristics to figure out whether this is a type alias or a regular variable. Things get even trickier when some items are not ready, because our heuristics must be robust w.r.t. to this.

I spent few hours today playing with this, and I don't think there is a fully-principled fix that doesn't involve rewriting a bunch of logic. But I think I have something that is still fairly principled. Namely, in this PR:

• Handle becomes_typeinfo status for imported placeholders consistently.
• Put should_wait_rhs() back in sync with can_be_type_alias() w.r.t. OpExpr.
• Add "backtracking" logic (maybe type alias -> actually a regular variable) for one rare case that is tricky to handle otherwise.

Note, the root cause of the crash is that we normally never change a type alias back to a regular variable. Normal flow for a symbol table entry is always like this None -> PlaceholderNode -> PlaceholderNode(becomes_typeinfo=True) -> SymbolNode (where some steps may be skipped). A lot of logic depends on this, but in the relevant scenarios it is tricky to follow this principle.

cc @JukkaL

[JukkaL]

Looks good, but this still causes a crash (not sure if this has be same root cause though):

class Test(Missing):
    A = 1
    B = 2

AB = Test.A | Test.B
Copy = AB
reveal_type(AB)
reveal_type(Copy)

[ilevkivskyi]

@JukkaL Oh this is funny, it was Saturday evening, and just before going to bed I remembered to remove the if isinstance(rvalue, OpExpr) when backtracking specifically because of such scenario, but then decided I will do it in the morning. As you can see I didn't (and likely would just merged it if you wouldn't notice).

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

prefect (https://github.com/PrefectHQ/prefect)
- src/prefect/settings/context.py:29: error: Variable "prefect.settings.context.SettingKey" is not valid as a type  [valid-type]
- src/prefect/settings/context.py:29: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
- src/prefect/settings/context.py:30: error: Variable "prefect.settings.context.SettingKey" is not valid as a type  [valid-type]
- src/prefect/settings/context.py:30: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
- src/prefect/settings/context.py:31: error: Variable "prefect.settings.context.SettingKey" is not valid as a type  [valid-type]
- src/prefect/settings/context.py:31: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases