Implement throwing an exception when using explicit API on graph in recording mode

[EwanC]

Implement specification defined behaviour for graph:add() around throwing an exception when used on a graph that's recording to a queue.

* Throws synchronously with error code `invalid` if a queue is recording
  commands to the graph.

The XFAIL in test https://github.com/reble/llvm/blob/sycl-graph-develop/sycl/test-e2e/Graph/Explicit/while_recording.cpp#L8 should be removed as part of implementing this behaviour.