[ty] Unpack variadic argument type in specialization (#20130)

## Summary

This PR fixes various TODOs around overload call when a variadic
argument is used.

The reason this bug existed is because the specialization wouldn't
account for unpacking the type of the variadic argument.

This is fixed by expanding `MatchedArgument` to contain the type of that
argument _only_ when it is a variadic argument. The reason is that
there's a split for when the argument type is inferred -- the
non-variadic arguments are inferred using `infer_argument_types` _after_
parameter matching while the variadic argument type is inferred _during_
the parameter matching. And, the `MatchedArgument` is populated _during_
parameter matching which means the unpacking would need to happen during
parameter matching.

This split seems a bit inconsistent but I don't want to spend a lot of
time on trying to merge them such that all argument type inference
happens in a single place. I might look into it while adding support for
`**kwargs`.

## Test Plan

Update existing tests by resolving the todos.

The ecosystem changes looks correct to me except for the `slice` call
but it seems that it's unrelated to this PR as we infer `slice[Any, Any,
Any]` for a `slice(1, 2, 3)` call on `main` as well
([playground](https://play.ty.dev/9eacce00-c7d5-4dd5-a932-4265cb2bb4f6)).

crates/ty_python_semantic/resources/mdtest/call/overloads.md

@@ -290,16 +290,10 @@ from overloaded import A, f
 
 def _(x: int, y: A | int):
     reveal_type(f(x))  # revealed: int
-    # TODO: revealed: int
-    # TODO: no error
-    # error: [no-matching-overload]
-    reveal_type(f(*(x,)))  # revealed: Unknown
+    reveal_type(f(*(x,)))  # revealed: int
 
     reveal_type(f(y))  # revealed: A | int
-    # TODO: revealed: A | int
-    # TODO: no error
-    # error: [no-matching-overload]
-    reveal_type(f(*(y,)))  # revealed: Unknown
+    reveal_type(f(*(y,)))  # revealed: A | int
 ```
 
 ### Generics (PEP 695)
@@ -328,16 +322,10 @@ from overloaded import B, f
 
 def _(x: int, y: B | int):
     reveal_type(f(x))  # revealed: int
-    # TODO: revealed: int
-    # TODO: no error
-    # error: [no-matching-overload]
-    reveal_type(f(*(x,)))  # revealed: Unknown
+    reveal_type(f(*(x,)))  # revealed: int
 
     reveal_type(f(y))  # revealed: B | int
-    # TODO: revealed: B | int
-    # TODO: no error
-    # error: [no-matching-overload]
-    reveal_type(f(*(y,)))  # revealed: Unknown
+    reveal_type(f(*(y,)))  # revealed: B | int
 ```
 
 ### Expanding `bool`
@@ -1236,21 +1224,14 @@ def _(integer: int, string: str, any: Any, list_any: list[Any]):
     reveal_type(f(*(integer, string)))  # revealed: int
 
     reveal_type(f(string, integer))  # revealed: int
-    # TODO: revealed: int
-    # TODO: no error
-    # error: [no-matching-overload]
-    reveal_type(f(*(string, integer)))  # revealed: Unknown
+    reveal_type(f(*(string, integer)))  # revealed: int
 
     # This matches the second overload and is _not_ the case of ambiguous overload matching.
     reveal_type(f(string, any))  # revealed: Any
-    # TODO: Any
-    reveal_type(f(*(string, any)))  # revealed: tuple[str, Any]
+    reveal_type(f(*(string, any)))  # revealed: Any
 
     reveal_type(f(string, list_any))  # revealed: list[Any]
-    # TODO: revealed: list[Any]
-    # TODO: no error
-    # error: [no-matching-overload]
-    reveal_type(f(*(string, list_any)))  # revealed: Unknown
+    reveal_type(f(*(string, list_any)))  # revealed: list[Any]
 ```
 
 ### Generic `self`