Java/C++/C#: Add field flow support for stores in nested fields.

[aschackmull]

This is essentially a one-line change to the field flow implementation, which enables tracking of stores into nested fields. In order for this to work, all of the associated nodes need to have corresponding PostUpdateNodes, so a few needed to be added for Java. The test case highlights the new possibilities for flow tracking.

[hvitved]

Nice!

[yh-semmle]

For reference, the result changes on JDK 8 were:

=== Changes per project (new / fixed) ===

jdk8-java-2                                                                14 / 7


=== Changes per query (new / fixed) ===

ArithmeticWithExtremeValues.ql                                             14 / 2
ConditionalBypass.ql                                                        0 / 4
ZipSlip.ql                                                                  0 / 1

[aschackmull]

I just sanity-checked the missing ZipSlip result - it's a FP and can be made to reappear by setting e.g. override int fieldFlowBranchLimit() { result = 10 }, so I'd say those result changes look fine.