[analyzer][CFG] Don't track the condition of asserts

Well, what is says on the tin I guess!

Some more changes:

* Move isInevitablySinking() from BugReporter.cpp to CFGBlock's interface
* Rename and move findBlockForNode() from BugReporter.cpp to
ExplodedNode::getCFGBlock()

Differential Revision: https://reviews.llvm.org/D65287

llvm-svn: 368836