C++: Add a new library for constructing macro flow

[MathiasVP]

This is a new cute little library I made as a response to a request from @bdrodes. The library makes it easy to construct paths that shows how an expression constructed from a set of nested macro expansions ends up in a piece of code. For example, let's say we want to find uses of sizeof(int) and we see code like this:

#define DEBUG 1

#define MY_SIZEOF(x) sizeof(x)

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

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

If you simply create a query that finds occurences of sizeof(int) you'll flag up the returned expression. However, it's not clear to a user why that expression is being alerted on. With this library we can do:

/**
 * @kind path-problem
 */

import cpp
import semmle.code.cpp.macroflow.MacroFlow

module MyConfig implements MacroFlow::ConfigSig {
  predicate isSink(Expr e) { e.(SizeofTypeOperator).getTypeOperand() instanceof IntType }
}

module Flow = MacroFlow::Make<MyConfig>;

import Flow::PathGraph

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

and we then get:

which makes it clear to a user where that sizeof is coming from.

I think this library could be really useful in general for C/C++ queries going forward. I may even upstream it to GitHub if I can find a good place to use it.