[ty] Lazily build TypeVar accumulations (#24782)
## Summary
Given a generic specialization, we were rebuilding the constraints after
every argument, rather than all-at-once.
E.g., for:
```python
def combine[T](a: T, b: T, c: T, d: T) -> T:
return a
combine(("name", 1), ("id", 2), ("flag", True), ("size", 4))
```
Each argument constrains the same type variable `T`:
```python
T = tuple[Literal["name"], Literal[1]]
T = tuple[Literal["id"], Literal[2]]
T = tuple[Literal["flag"], Literal[True]]
T = tuple[Literal["size"], Literal[4]]
```
On main, we then compute (roughly):
```
T = A
T = union(A, B)
T = union(union(A, B), C)
T = union(union(A, B, C), D)
```
Now, we create a builder and construct at the end. This has a
significant impact on functions with many arguments, but also reduces
memory on real-world projects, which is great.
