[OpenMP] Diagnose missing cases of statements between target and teams directives

Clang fails to diagnose cases such as
#pragma omp target
  while(0) {
    #pragma omp teams
    {}
  }

A patch by David Sheinkman.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@273908 91177308-0d34-0410-b5e6-96231b3b80d8