Clang to flowchart

I want to produce some flowcharts and workflows from a complex
multit-hreaded C++ codebase.

I was thinking that gcc-xml might be useful so I asked on the mailing
list, and they suggested that Clang might be a useful option.

Are there any tools based on Clang that can produce flowcharts/workflows
so I can get a better picture of how the complex multit-hreaded C++
operates ??

Is there suitable output from Clang about all the function calls, etc
(e.g. in xml format) that can be used to generate UML or flowcharts or
other documents ??

Thanks, Brendan.