[ty] Support empty function bodies in if TYPE_CHECKING blocks (#19372)

## Summary

Resolves https://github.com/astral-sh/ty/issues/339

Supports having a blank function body inside `if TYPE_CHECKING` block or
in the elif or else of a `if not TYPE_CHECKING` block.

```py
if TYPE_CHECKING:
    def foo() -> int: ...

if not TYPE_CHECKING: ...
else:     
    def bar() -> int: ...
```

## Test Plan

Update `function/return_type.md`

---------

Co-authored-by: Carl Meyer <carl@astral.sh>

crates/ty_python_semantic/resources/mdtest/function/return_type.md

@@ -127,6 +127,80 @@ def f(x: int | str):
     return x
 ```
 
+### In `if TYPE_CHECKING` block
+
+Inside an `if TYPE_CHECKING` block, we allow "stub" style function definitions with empty bodies,
+since these functions will never actually be called.
+
+```py
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    def f() -> int: ...
+
+else:
+    def f() -> str:
+        return "hello"
+
+reveal_type(f)  # revealed: def f() -> int
+
+if not TYPE_CHECKING:
+    ...
+elif True:
+    def g() -> str: ...
+
+else:
+    def h() -> str: ...
+
+if not TYPE_CHECKING:
+    def i() -> int:
+        return 1
+
+else:
+    def i() -> str: ...
+
+reveal_type(i)  # revealed: def i() -> str
+
+if False:
+    ...
+elif TYPE_CHECKING:
+    def j() -> str: ...
+
+else:
+    def j_() -> str: ...  # error: [invalid-return-type]
+
+if False:
+    ...
+elif not TYPE_CHECKING:
+    def k_() -> str: ...  # error: [invalid-return-type]
+
+else:
+    def k() -> str: ...
+
+class Foo:
+    if TYPE_CHECKING:
+        def f(self) -> int: ...
+
+if TYPE_CHECKING:
+    class Bar:
+        def f(self) -> int: ...
+
+def get_bool() -> bool:
+    return True
+
+if TYPE_CHECKING:
+    if get_bool():
+        def l() -> str: ...
+
+if get_bool():
+    if TYPE_CHECKING:
+        def m() -> str: ...
+
+if TYPE_CHECKING:
+    if not TYPE_CHECKING:
+        def n() -> str: ...
+```
+
 ## Conditional return type
 
 ```py