Fix missing meet case exposed by len narrowing (#16470)

Fixes #16468

The fix is straightforward.

Btw when fixing this I noticed that we disregard type arguments when
narrowing, for example:
```python
x: Sequence[int]
if isinstance(x, tuple):
    reveal_type(x)  # tuple[Any, ...], but should be `tuple[int, ...]`
```
I guess fixing this may be tricky, and it is quite old behavior.

mypy/meet.py

@@ -16,6 +16,7 @@
 from mypy.typeops import is_recursive_pair, make_simplified_union, tuple_fallback
 from mypy.types import (
     MYPYC_NATIVE_INT_NAMES,
+    TUPLE_LIKE_INSTANCE_NAMES,
     AnyType,
     CallableType,
     DeletedType,
@@ -936,7 +937,7 @@ def visit_tuple_type(self, t: TupleType) -> ProperType:
             return TupleType(items, tuple_fallback(t))
         elif isinstance(self.s, Instance):
             # meet(Tuple[t1, t2, <...>], Tuple[s, ...]) == Tuple[meet(t1, s), meet(t2, s), <...>].
-            if self.s.type.fullname == "builtins.tuple" and self.s.args:
+            if self.s.type.fullname in TUPLE_LIKE_INSTANCE_NAMES and self.s.args:
                 return t.copy_modified(items=[meet_types(it, self.s.args[0]) for it in t.items])
             elif is_proper_subtype(t, self.s):
                 # A named tuple that inherits from a normal class

test-data/unit/check-narrowing.test

@@ -1910,3 +1910,16 @@ if len(x) == a:
 else:
     reveal_type(x)  # N: Revealed type is "Union[Tuple[builtins.int, builtins.int], Tuple[builtins.int, builtins.int, builtins.int]]"
 [builtins fixtures/len.pyi]
+
+[case testNarrowingLenUnionWithUnreachable]
+from typing import Union, Sequence
+
+def f(x: Union[int, Sequence[int]]) -> None:
+    if (
+        isinstance(x, tuple)
+        and len(x) == 2
+        and isinstance(x[0], int)
+        and isinstance(x[1], int)
+    ):
+        reveal_type(x)  # N: Revealed type is "Tuple[builtins.int, builtins.int]"
+[builtins fixtures/len.pyi]

test-data/unit/fixtures/len.pyi

@@ -10,7 +10,7 @@ class object:
 class type:
     def __init__(self, x) -> None: pass
 
-class tuple(Generic[T]):
+class tuple(Sequence[T]):
     def __len__(self) -> int: pass
 
 class list(Sequence[T]): pass