C++: Implement MaD support for flow through perfect-forwarding functions

[MathiasVP]

This PR implements the necessary library changes to support MaD summaries for functions such as vector::emplace_back or make_unique. These functions receive a list of arguments and then forwards them to a constructor call.

@hvitved had a super cool implementation idea. Given:

struct Foo {
  Foo(int);
};

std::vector<Foo> v;

we model a call such as v.emplace_back(42) as:

v.emplace_back(42, &Foo)

and give emplace_back two summaries:

1. Argument[0] -> Argument[1].Parameter[0]
2. Argument[1].Parameter[this] -> Argument[this].Element

The first summary states that 42 goes into the 0'th parameter of the Foo constructor, and the second summary states that the this parameter of the constructed object goes into the this argument of v with an Element content.

(A few lines I told in the above paragraph:

• You cannot take the address of a constructor in C++. But that doesn't mean we cannot use it as an implementation detail!
• We don't actually use the last argument as the argument position of the synthetic function pointer for the constructor. This PR adds a new argument position which we name forward in MaD. I'm happy to change this name to something else if anyone has any strong opinions about this.)

In order to know which constructor to forward to we need to know what type is being constructed. For example:

std::vector<Foo> v;
v.emplace_back(42); // calls `Foo(42)`
v.emplace(v.begin(), 42); // calls `Foo(42)`
std::make_unique<Foo> p(42); // calls `Foo(42)`

To know which type is constructed we add a new extensible called forwardsModel with rows very much like what we have for MaD summaries. For example, I've added this as row as a test:

["", "Container<T>", True, "emplace<Args>", "(Args &&)", "", "0", "T", "manual"]

this says that a call to Container<T>::emplace(args0, ..., argsN) forwards its arguments to a call to T(args0, ..., argsN). The 0 specifies an offset so we can support cases like v.emplace(v.begin(), 42) where we need to ignore the first argument.

This PR doesn't actually add any non-test MaD summaries. I'll delay that to a future PR.