Generate a callsite identifier associated with each callsite

1. Generate a unique string name for a callsite that appears in every node
2. This identifier will refer to a single callsite and is used for trace data retrieval

@ke-yu
@lukechurch
@Steell
@Benglin

'This is the count in which this function appears lexically in the program'

I don't quite understand this.

Do you mean "this is the count of the number of times in which this callsite appears in the program'?