Red Knot - Add symbol flags (#11134)
* Adds `Symbol.flag` bitfield. Populates it from (the three renamed)
`add_or_update_symbol*` methods.
* Currently there are these flags supported:
* `IS_DEFINED` is set in a scope where a variable is defined.
* `IS_USED` is set in a scope where a variable is referenced. (To have
both this and `IS_DEFINED` would require two separate appearances of a
variable in the same scope-- one def and one use.)
* `MARKED_GLOBAL` and `MARKED_NONLOCAL` are **not yet implemented**.
(*TODO: While traversing, if you find these declarations, add these
flags to the variable.*)
* Adds `Symbol.kind` field (commented) and the data structure which will
populate it: `Kind` which is an enum of freevar, cellvar,
implicit_global, and implicit_local. **Not yet populated**. (*TODO: a
second pass over the scope (or the ast?) will observe the
`MARKED_GLOBAL` and `MARKED_NONLOCAL` flags to populate this field. When
that's added, we'll uncomment the field.*)
* Adds a few tests that the `IS_DEFINED` and `IS_USED` fields are
correctly set and/or merged:
* Unit test that subsequent calls to `add_or_update_symbol` will merge
the flag arguments.
* Unit test that in the statement `x = foo`, the variable `foo` is
considered used but not defined.
* Unit test that in the statement `from bar import foo`, the variable
`foo` is considered defined but not used.
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Co-authored-by: Carl Meyer <carl@astral.sh>
