[DivergenceAnalysis] Separated definition of class into header.

The definition of the DivergenceAnalysis pass was in a CPP
file and wasn't accessible to users of the analysis to get it
through "getAnalysis<>()".
This patch extracts the definition into a separate header that
can be used by users of the analysis to fetch the results.

Patch by Volkan Keles (vkeles@apple.com)

llvm-svn: 248186