[OPENMP] OMPExecutableDirective re-factoring

Store the number of clauses and children of OMPExecutableDirective and dynamically compute the locations of corresponding arrays.

http://llvm-reviews.chandlerc.com/D2977



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204933 91177308-0d34-0410-b5e6-96231b3b80d8