[OpenMP] Parsing and sema support for the to clause

Summary:
The patch contains the parsing and sema support for the `to` clause. 

Patch based on the original post by Kelvin Li.

Reviewers: carlo.bertolli, hfinkel, kkwli0, arpith-jacob, ABataev

Subscribers: caomhin, cfe-commits

Differential Revision: http://reviews.llvm.org/D18597

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@270880 91177308-0d34-0410-b5e6-96231b3b80d8