This changes the API graph label representing the receiver of call from parameter -1 to receiver.

The member predicates of API::Node are affected as follows:

• getParameter(i) no longer has a result for i = -1, previously it returned the receiver.
• getAParameter() no longer includes the receiver.
• getLastParameter() no longer returns the receiver when the call has zero arguments. (🤦)

In theory this is not a behaviour-preserving change, but in practice it doesn't change much:

• There are 90 uses of getParameter(i) outside ApiGraphs.qll, all of them unaffected due to how they restrict i.
• There are 11 uses of getAParameter, some of which are affected, but as far as I can tell, none of them actually intended to get the receiver.
• There are 3 uses of getLastParameter, which are affected in theory but unlikely to matter in practice (since the modeled function expects >=1 arguments), and none of them intended to get the receiver.

This was motivated by an upcoming change in MaD, where we want to use Argument[this] to represent the receiver (Argument[self] in Python/Ruby).

Evaluation looks good. A few spurious taint sources have been removed, which referred to the receiver argument to a callback.