[ty] Semantic tokens: mark comprehension targets as definitions

[lucach]

Summary

In #21521 I went through all the statements that may introduce names and marked those names as definitions when computing the semantic tokens. However, names can also be introduced by expressions. Some were already covered (lambdas, named expressions), but comprehensions/generators were not. This commit fixes that, and hopefully completes the set of places that can introduce names.

Test Plan

Added a test to check that the targets of list/set/dict comprehensions and generator expressions are marked as definitions.

[astral-sh-bot]

Diagnostic diff on typing conformance tests

No changes detected when running ty on typing conformance tests ✅

[astral-sh-bot]

mypy_primer results

Changes were detected when running on open source projects

dd-trace-py (https://github.com/DataDog/dd-trace-py)
- ddtrace/appsec/_handlers.py:404:13: error[invalid-argument-type] Argument to bound method `add_configurations` is incorrect: Expected `list[tuple[str, str, str]]`, found `list[Unknown | tuple[str, int, Unknown]] & list[tuple[str, str, str] | tuple[str, int, Unknown]]`
+ ddtrace/appsec/_handlers.py:404:13: error[invalid-argument-type] Argument to bound method `add_configurations` is incorrect: Expected `list[tuple[str, str, str]]`, found `list[tuple[str, str, str] | tuple[str, int, Unknown]] & list[Unknown | tuple[str, int, Unknown]]`

No memory usage changes detected ✅

[MichaReiser]

Thank you