C# 14: Null conditional assignments.

[michaelnebel]

In this PR, we:

• Add support for C# 14 null conditional assignments. While the extractor already supported this feature, updates to the C# control flow implementation were needed.
• Simplify the control flow logic for qualified write access, which also helps with the addition of null conditional assignments.
• Make a small improvement to the MaybeNullExpr class so that it now correctly handles null conditional (read) accesses.

Changes to existing control flow

Previously, the control flow for

ca.IntField = 42

looked like this:

ca -> 42 -> ca.IntField = 42

With this PR, we now include the field access after evaluating the right-hand side, so the control flow becomes:

ca -> 42 -> ca.IntField -> ca.IntField = 42

This matches how property assignments are handled, where it’s important for the property access to be included in the control flow after evaluating the right-hand side, since accessing the property corresponds to invoking the property setter.

New control flow.

The changes to the control flow graph implementation mean that for a statement like

ca?.Prop?.StringProp = "World";

the control flow is now built correctly to reflect the sequence of null conditional accesses.

That is, if the qualifier uses a null conditional, we add a null edge from the qualifier to the point after the assignment. This reflects the fact that if the qualifier is null, the right-hand side of the assignment isn’t evaluated.