[OPENMP] Loop canonical form analysis (Sema)

This patch implements semantic analysis to make sure that the loop is in OpenMP canonical form.
This is the form required for 'omp simd', 'omp for' and other loop pragmas.

Differential revision: http://reviews.llvm.org/D3778



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210095 91177308-0d34-0410-b5e6-96231b3b80d8