[StaticAnalyzer] Completely unrolling specific loops with known bound option

[StaticAnalyzer] Completely unrolling specific loops with known bound option

This feature allows the analyzer to consider loops to completely unroll.
New requirements/rules (for unrolling) can be added easily via ASTMatchers.

Right now it is hidden behind a flag, the aim is to find the correct heuristic
and create a solution which results higher coverage % and more precise
analysis, thus can be enabled by default.

Right now the blocks which belong to an unrolled loop are marked by the
LoopVisitor which adds them to the ProgramState.
Then whenever we encounter a CFGBlock in the processCFGBlockEntrance which is
marked then we skip its investigating. That means, it won't be considered to
be visited more than the maximal bound for visiting since it won't be checked.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@309006 91177308-0d34-0410-b5e6-96231b3b80d8