C#: Introduce inherited CFG completions

[hvitved]

When completions are inherited by elements inside finally blocks, we previously
threw away the underlying completion. For example, in

try
{
    if (b)
        throw new Exception();
}
finally
{
    if (b)
        ...
}

the completions for b inside the finally block are true and throw(Exception),
where the latter is inherited from the try block, with an underlying false
completion. Throwing away the false completion meant that we were unable to prune
the false edge (Boolean CFG splitting).

Profiling report here (internal link).

[calumgrant]

As discussed offline, we agreed that NestedCompletion, Nested* , Nestable* would be slightly better, and also getInnerCompletion() and getOuterCompletion().

I wasn't sure about the need for the split the completion classes into -Inherited and -Direct (perhaps renamed to Nested- and NonNested-). I think it would be more maintainable to avoid the Nested- variants (and assume everything is NonNested-), and remember to use getInnerCompletion() where needed.

[calumgrant]

LGTM and happy to merge. A few very minor comments that could be fixed later/never if you prefer.

[calumgrant]

We should remove the word "inherits" and "underlying" and replace it with "has the outer" and "has the inner"