Replace some assert(0)'s with llvm_unreachable.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211139 91177308-0d34-0410-b5e6-96231b3b80d8
[OPENMP] Initial support for '#pragma omp for' (fixed incompatibility with MSVC).


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211140 91177308-0d34-0410-b5e6-96231b3b80d8