[analyzer] ConditionBRVisitor: Unknown condition evaluation support

Summary:
If the constraint information is not changed between two program states the
analyzer has not learnt new information and made no report. But it is
possible to happen because we have no information at all. The new approach
evaluates the condition to determine if that is the case and let the user
know we just `Assuming...` some value.

Reviewers: NoQ, george.karpenkov

Reviewed By: NoQ

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

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D57410

llvm-svn: 356323