Support OpenMP version 4.5 syntax for #pragma omp declare target.

Syntax:
  #pragma omp declare target (extended-list) new-line
or
  #pragma omp declare target clause[ [,] clause ... ] new-line

Where clause is one of the following:
  to(extended-list)
  link(list)

Differential Revision: http://reviews.llvm.org/D20011

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@268925 91177308-0d34-0410-b5e6-96231b3b80d8