Unexpected "unreachable code" warning for `const` used by a local function

```cs
f();
return;

const string s = "a"; // warning CS0162: Unreachable code detected

void f()
{
System.Console.Write(s);
}
```

The program outputs "a", i.e., the `const` has been actually used, so the "unreachable code" warning is incorrect.

Start by running the minimal C# reproduction from the issue and confirming warning CS0162 while the program outputs "a". Trace the compiler's unreachable-code analysis for the local function and const, and consider the issue done when this valid usage no longer produces the incorrect warning without regressing genuine unreachable-code diagnostics.

Written by the indexing model from the issue text.