[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

llvm-svn: 270880