This allows a member initializer list to be seen as a sequence of field
assignments. For example, the constructor
C() : a(taint()) { }
now has data flow similar to
C() { this.a = taint(); }
This commit changes C++ `ConstructorCall` to behave like
`new`-expressions in Java: they are both `ExprNode`s and
`PostUpdateNodes`, and there's a "pre-update node" (here called
`PreConstructorCallNode`) to play the role of the qualifier argument
when calling a constructor.
