C#: Remove expanded assignments.

[michaelnebel]

This PR has two goals, primarily to avoid making multiple upgrade/downgrade scripts:

• Remove expanded assignments. This aligns our implementation with Java and should make adoption of the shared CFG library easier. It also simplifies expression handling and avoids caching a large predicate. Previously, when the extractor encountered a += b, it would synthesize a = a + b.
• Swap the left- and right-hand-side child indices for assignments. This lets us remove several hacks/rotations currently used in the code.

Review on a commit-by-commit basis is encouraged.

Implementation notes

• The implementation now “correctly” classifies a += b (e.g., when a and b are int) as an operator call, since it implicitly invokes the user-defined static operator on Int32.
• The only compound assignment that does not call an operator is ??= as the null-coalescing operator is a built-in short-circuit like operation.
• Added a new set of classes to represent expressions involving a specific operation (e.g., AddOperation, which can represent either a + b or a += b).

DCA notes

• Performance appears unchanged.
• The missing result for cs/unsynchronized-getter is due to removal of a false positive: the query did not properly account for expanded assignments.
• The missing result for cs/web/xss looks acceptable: one alert is removed, but it is essentially a shorter path to the same underlying alert (which is here)
• The new results for cs/useless-upcast look acceptable.
• Remaining differences appear consistent with normal DCA background noise.

[michaelnebel]

@hvitved : I would really appreciate a review of this PR even though it is still in draft mode. The PR still needs upgrade/downgrade scripts and more DCA runs.