[freddyuniverse]

Fixes #22265.

When mypy checks a comprehension, it assigns each item type to the index
variable. Any error from that assignment, such as Need type annotation
or an unpacking error, was reported at the start of the whole
comprehension. That's the wrong line for a multi-line comprehension. For
a generator expression, the line also depended on the parser: the old
parser reported the sum( line and the native parser reported the
element line.

This PR passes the index expression as the error context in
check_for_comp, so the error lands on the for ... in ... clause that
defines the variable. With nested for clauses, each error now points
at its own clause. Single-line comprehensions report on the same line as
before; only the column changes, to the variable.

Tests: I added testInferenceOfIndexInMultiLineComprehension and an
_old_parser copy to check-inference.test. They cover a generator
passed to a call, list/set/dict comprehensions, a nested for clause,
and an unpacking error. Both fail on master and pass with the change.
No existing expected outputs needed updating.

Locally (Python 3.13, Linux):

• testcheck, testfinegrained, testsemanal, testtypegen,
  testerrorstream, testmerge, testdeps: 10010 passed, 56 skipped,
  7 xfailed
• testpythoneval, testcmdline, testdaemon, testreports: 346
  passed, 1 xfailed
• python runtests.py self: no issues
• pre-commit run on the changed files (black, ruff, codespell): passed

One user-visible effect: a # type: ignore placed on the first line of a
multi-line comprehension to silence one of these errors would now be
unused, and the error would show up on the for line instead.

I used an AI assistant while working on this. I reviewed the change and
the tests myself and can answer questions about them.

[github-actions]

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

prefect (https://github.com/PrefectHQ/prefect)
- src/prefect/utilities/collections.py:507: error: "None" object is not iterable  [misc]
+ src/prefect/utilities/collections.py:508: error: "None" object is not iterable  [misc]
- src/prefect/server/events/storage/__init__.py:80: error: "int" object is not iterable  [misc]
+ src/prefect/server/events/storage/__init__.py:88: error: "int" object is not iterable  [misc]

jax (https://github.com/google/jax)
- jax/_src/api_util.py:769: error: Need type annotation for "path"  [var-annotated]
+ jax/_src/api_util.py:771: error: Need type annotation for "path"  [var-annotated]

ibis (https://github.com/ibis-project/ibis)
- ibis/backends/sql/datatypes.py:1794: error: Need type annotation for "mapper"  [var-annotated]
+ ibis/backends/sql/datatypes.py:1796: error: Need type annotation for "mapper"  [var-annotated]
- ibis/expr/types/strings.py:973: error: Need type annotation for "pattern"  [var-annotated]
+ ibis/expr/types/strings.py:975: error: Need type annotation for "pattern"  [var-annotated]
- ibis/expr/types/strings.py:1019: error: Need type annotation for "pattern"  [var-annotated]
+ ibis/expr/types/strings.py:1021: error: Need type annotation for "pattern"  [var-annotated]
- ibis/expr/types/relations.py:1141: error: Need type annotation for "arg"  [var-annotated]
+ ibis/expr/types/relations.py:1143: error: Need type annotation for "arg"  [var-annotated]

sphinx (https://github.com/sphinx-doc/sphinx)
+ sphinx/testing/util.py:80:1: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports

static-frame (https://github.com/static-frame/static-frame)
- static_frame/core/frame.py:7193: error: Need type annotation for "name"  [var-annotated]
- static_frame/core/frame.py:7436: error: Need type annotation for "name"  [var-annotated]

scipy-stubs (https://github.com/scipy/scipy-stubs)
- tests/misc/test_submodules_empty.pyi:3: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports