[DOTGraphTraits] Propagate Graph template argument, NFC

Propagates the GraphT template argument to the default value of
the AnalysisGraphTraitsT template argument. This allows to specialize
the DefaultAnalysisGraphTraits<AnalysisT,GraphT> for analysis with a
graph type different from the analysis type and it will automatically
get picked-up.

Note: This was probably the intended purpose and should not result in any
      functional change.
llvm-svn: 308878