[OpenMP] Add support for the 'private pointer' flag to signal variables...

[OpenMP] Add support for the 'private pointer' flag to signal variables captured in target regions and used in first-private clauses.

Summary: If a variable is implicitly mapped (doesn't show in a map clause), the runtime library has to be informed if the corresponding capture shows up in first-private clause, so that the storage previously allocated in the device is used. This patch adds the support for that.

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

Subscribers: caomhin, cfe-commits

Differential Revision: http://reviews.llvm.org/D20112

llvm-svn: 270870