[PM] Add a really simple trait to the DOTGraphTraitsPass class templates

[PM] Add a really simple trait to the DOTGraphTraitsPass class templates
that lets the analysis and graph types be separate and the graph
computed from the analysis through some arbitrary user-supplied code.

This will allow a call graph to an independent entity from the pass
which creates it which is necessary for the new pass manager.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195717 91177308-0d34-0410-b5e6-96231b3b80d8