[OpenMP] Fix parsing of delete map clause modifier in C++ mode.

Summary: The map modifier 'delete' is parser in c++ mode as a delete keyword, which requires special handling in the map clause parsing.

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

Subscribers: cfe-commits, fraggamuffin, caomhin

Differential Revision: http://reviews.llvm.org/D17629

llvm-svn: 262094