Fix handling of NoReturn in Union return types (#11996)

There are several discussions and comments describing the following problem 
(references can be found at the end of the PR summary):

```python
def func() -> str | NoReturn:
    ...

func().lower()
```

At the moment the code results in: `"NoReturn" of "Union[str, NoReturn]" has no 
attribute "lower"`

Make `Union[int, NoReturn]` equivalent to `int` in a return type, because in case 
the function returns it must be `int`.

mypy/checkexpr.py

@@ -378,6 +378,8 @@ def visit_call_expr_inner(self, e: CallExpr, allow_none_return: bool = False) ->
         if isinstance(e.callee, MemberExpr) and e.callee.name == 'format':
             self.check_str_format_call(e)
         ret_type = get_proper_type(ret_type)
+        if isinstance(ret_type, UnionType):
+            ret_type = make_simplified_union(ret_type.items)
         if isinstance(ret_type, UninhabitedType) and not ret_type.ambiguous:
             self.chk.binder.unreachable()
         # Warn on calls to functions that always return None. The check

test-data/unit/check-unions.test

@@ -144,6 +144,11 @@ f(1)
 f(None)
 f('') # E: Argument 1 to "f" has incompatible type "str"; expected "Optional[int]"
 
+[case testUnionWithNoReturn]
+from typing import Union, NoReturn
+def f() -> Union[int, NoReturn]: ...
+reveal_type(f()) # N: Revealed type is "builtins.int"
+
 [case testUnionSimplificationGenericFunction]
 from typing import TypeVar, Union, List
 T = TypeVar('T')