[Analyzer] Terminate analysis on OpenMP code instead of assertion crash

* ExprEngine assumes that OpenMP statements should never appear in CFG.
  However, current CFG doesn't know anything about OpenMP and passes
  such statements as CFG nodes causing "UNREACHABLE executed!" crashes.
  Since there is no OpenMP implementation in ExprEngine or CFG,
  we stop the analysis on OpenMP statements to avoid crashes.

This fixes PR31835.

Differential Revision: https://reviews.llvm.org/D30565

llvm-svn: 296884