[OpenMP][Docs] Provide implementation status details

This adds a more fine-grained list of OpenMP features with their
implementation status and associated reviews/commits.

Reviewers: kkwli0, ABataev, RaviNarayanaswamy, gtbercea, Hahnfeld

Subscribers: bollu, guansong, jfb, hfinkel, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64375

llvm-svn: 370930

Author: Johannes Doerfert