Prevent TypeGuardedType leak from narrow_declared_type as part of typevar bound

[sterliakov]

Fixes #20015, refs #18895 as a previous example of the same issue. I don't see any similar problems in other branches.

[github-actions]

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

steam.py (https://github.com/Gobot1234/steam.py)
- steam/ext/commands/commands.py:285: error: Incompatible types in assignment (expression has type "GroupMixin[Any | None] | Group[Any, [VarArg(Any), KwArg(Any)], Any] | None", variable has type "Self")  [assignment]
+ steam/ext/commands/commands.py:285: error: Incompatible types in assignment (expression has type "GroupMixin[Any | None] | Group[CogT, [VarArg(Any), KwArg(Any)], Any] | None", variable has type "Self")  [assignment]

scikit-build-core (https://github.com/scikit-build/scikit-build-core)
+ src/scikit_build_core/metadata/__init__.py:82: error: Argument 1 has incompatible type "Union[str, dict[str, str]]"; expected "str"  [arg-type]
+ src/scikit_build_core/metadata/__init__.py:82: note: Error code "arg-type" not covered by "type: ignore" comment
+ src/scikit_build_core/metadata/__init__.py:89: error: Argument 1 has incompatible type "Union[str, list[str], dict[str, str]]"; expected "str"  [arg-type]
+ src/scikit_build_core/metadata/__init__.py:89: note: Error code "arg-type" not covered by "type: ignore" comment
+ src/scikit_build_core/metadata/__init__.py:94: error: Item "str" of "Union[str, dict[str, str]]" has no attribute "items"  [union-attr]
+ src/scikit_build_core/metadata/__init__.py:98: error: Item "str" of "Union[str, dict[str, str]]" has no attribute "items"  [union-attr]
+ src/scikit_build_core/metadata/__init__.py:98: note: Error code "union-attr" not covered by "type: ignore" comment
+ src/scikit_build_core/metadata/__init__.py:102: error: Left operand of "and" is always true  [redundant-expr]
+ src/scikit_build_core/metadata/__init__.py:108: error: Item "str" of "Union[str, list[str], dict[str, str]]" has no attribute "items"  [union-attr]
+ src/scikit_build_core/metadata/__init__.py:108: error: Item "list[str]" of "Union[str, list[str], dict[str, str]]" has no attribute "items"  [union-attr]

[sterliakov]

All scikit_build_core errors are correct, both arg-type and return-value should be there.

[ilevkivskyi]

LG, thanks!