[red-knot] Avoid unresolved-reference in unreachable code (#17169)

## Summary

This PR changes the inferred type for symbols in unreachable sections of
code to `Never` (instead of reporting them as unbound), in order to
silence false positive diagnostics. See the lengthy comment in the code
for further details.

## Test Plan

- Updated Markdown tests.
- Manually verified a couple of ecosystem diagnostic changes.

crates/red_knot_python_semantic/resources/mdtest/terminal_statements.md

@@ -654,9 +654,7 @@ def f(cond: bool) -> str:
         reveal_type(x)  # revealed: Literal["before"]
         return "a"
         x = "after-return"
-        # TODO: no unresolved-reference error
-        # error: [unresolved-reference]
-        reveal_type(x)  # revealed: Unknown
+        reveal_type(x)  # revealed: Never
     else:
         x = "else"
     return reveal_type(x)  # revealed: Literal["else"]

crates/red_knot_python_semantic/resources/mdtest/unreachable.md

@@ -211,9 +211,6 @@ def f():
 
     print("unreachable")
 
-    # TODO: we should not emit an error here; we currently do, since there is no control flow path from this
-    # use of 'x' to any definition of 'x'.
-    # error: [unresolved-reference]
     print(x)
 ```
 
@@ -228,8 +225,6 @@ def outer():
     x = 1
 
     def inner():
-        # TODO: we should not emit an error here
-        # error: [unresolved-reference]
         return x  # Name `x` used when not defined
     while True:
         pass