LGTM.com - false positive - cs/useless-assignment-to-local when assigned as an out within a try/finally block

**Description of the false positive**

In general, the `cs/useless-assignment-to-local` rule makes sense: https://lgtm.com/rules/1506093386171/

However, in some cases, the error results in a false positive, where the compiler would throw if the assignment was removed. This happens if the local is assigned within a try/finally block (say, by a method call returning an `out`), and then used in the finally block. If the method call throws an exception, the local could be left uninitialized.

Here's a simplified example:
```C#
private void Test()
{
ArraySegment rented = default; // necessary, otherwise compiler complains

try
{
MethodThatMightThrow(out rented);
}
finally
{
DoSomething(rented);
}
}

private void MethodThatMightThrow(out ArraySegment rented)
{
//rented = default;
throw new InvalidOperationException();
}

private int DoSomething(ArraySegment rented)
{
return rented.Array.Length; // null ref if rented isn't assigned.
}
```

**URL to the alert on the project page on LGTM.com**

https://lgtm.com/projects/g/dotnet/corefx/latest/files/src/System.Text.Json/src/System/Text/Json/Document/JsonDocument.cs?sort=name&dir=ASC&mode=heatmap#V898