CS converter doesn't produce threadFlowLocation.location

[ghost]

When the CS converter translates a sequence of propagation-event elements into a threadFlow, it populates each threadFlowLocation.stack from the propagation-event's stack sub-element. The converter also takes the propagation-event's signature sub-element and places it at the top of the stack.

But the converter does not populate threadFlowLocation.location, which confuses the VS Code Viewer, which expects location to be present. Sure, we could "fix" the viewer, but really, signature does specify the location, and we should populate it.

@michaelcfanning @rscrivens FYI