[analyzer] ConditionBRVisitor: Enhance to write out more information

Summary:
Add extra messages to the bug report to inform the user why the analyzer
`Taking true/false branch`.

Reviewers: NoQ, george.karpenkov

Reviewed By: NoQ

Subscribers: gerazo, gsd, dkrupp, whisperity, baloghadamsoftware, xazax.hun,
             eraman, szepet, a.sidorin, mikhail.ramalho, Szelethus,
             donat.nagy, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D53076

llvm-svn: 362020