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

Is this call site identifier unique for each function call or each function? For the latter do we need to deal with overloaded function (because the parameter of the second GenerateCallsiteIdentifierForGraphNode() is just function name). If we need to take overloaded function into account, then we shouldn't have overloaded function. Besides, that null check could be removed, I think passing null to a function is caller's fault... :-)