C++: Support macro flow when the macro generates a child expression o…

[MathiasVP]

#304 added a new library for "macro flow". (See that description for what the library does.) Taking the example from that PR and changing it to:

#define DEBUG 1

#define MY_SIZEOF(x) sizeof(x)

#ifdef DEBUG
#define MY_FOO(y) MY_SIZEOF(y) + 1
#else
#define MY_FOO(y) 0
#endif

int test() {
  return MY_FOO(int);
}

No path is generated since MY_FOO(int) doesn't expand to exactly sizeof(int), but rather to a larger expression of which sizeof(int) is a subexpression.

With this PR we can now write:

// ...
from Flow::Node n1, Flow::Node n2, Expr e
where Flow::flowsTo(n1, n2, e, true)
select e, n1, n2, "Use of sizeof with integer type."

to keep the old behavior (i.e., only show paths when the expanded expression is exactly sizeof(int)), but we can also change true to _ to allow paths to only be generated when the sizeof(int) is a subexpression of the expanded macro.