[OpenMP] Parsing and sema support for the from clause

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

Patch based on the original post by Kelvin Li.

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

Subscribers: caomhin, cfe-commits

Differential Revision: http://reviews.llvm.org/D18488

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@270882 91177308-0d34-0410-b5e6-96231b3b80d8