[ty] Apply function specialization to all overloads (#18020)

Function literals have an optional specialization, which is applied to
the parameter/return type annotations lazily when the function's
signature is requested. We were previously only applying this
specialization to the final overload of an overloaded function.

This manifested most visibly for `list.__add__`, which has an overloaded
definition in the typeshed:


b398b83631/crates/ty_vendored/vendor/typeshed/stdlib/builtins.pyi (L1069-L1072)

Closes https://github.com/astral-sh/ty/issues/314

crates/ty_python_semantic/resources/mdtest/generics/legacy/classes.md

@@ -454,6 +454,28 @@ reveal_type(d.method3())  # revealed: SomeProtocol[int]
 reveal_type(d.method3().x)  # revealed: int
 ```
 
+When a method is overloaded, the specialization is applied to all overloads.
+
+```py
+from typing import overload, Generic, TypeVar
+
+S = TypeVar("S")
+
+class WithOverloadedMethod(Generic[T]):
+    @overload
+    def method(self, x: T) -> T:
+        return x
+
+    @overload
+    def method(self, x: S) -> S | T:
+        return x
+
+    def method(self, x: S | T) -> S | T:
+        return x
+
+reveal_type(WithOverloadedMethod[int].method)  # revealed: Overload[(self, x: int) -> int, (self, x: S) -> S | int]
+```
+
 ## Cyclic class definitions
 
 ### F-bounded quantification

crates/ty_python_semantic/resources/mdtest/generics/pep695/classes.md

@@ -347,6 +347,26 @@ reveal_type(c.method2())  # revealed: str
 reveal_type(c.method3())  # revealed: LinkedList[int]
 ```
 
+When a method is overloaded, the specialization is applied to all overloads.
+
+```py
+from typing import overload
+
+class WithOverloadedMethod[T]:
+    @overload
+    def method(self, x: T) -> T:
+        return x
+
+    @overload
+    def method[S](self, x: S) -> S | T:
+        return x
+
+    def method[S](self, x: S | T) -> S | T:
+        return x
+
+reveal_type(WithOverloadedMethod[int].method)  # revealed: Overload[(self, x: int) -> int, (self, x: S) -> S | int]
+```
+
 ## Cyclic class definitions
 
 ### F-bounded quantification