[OpenMP] Add fields for flags in the offload entry descriptor.

Summary:
This patch adds two fields to the offload entry descriptor. One field is meant to signal Ctors/Dtors and `link` global variables, and the other is reserved for runtime library use. 

 Currently, these fields are only filled with zeros in the current code generation, but that will change when `declare target` is added. 

The reason, we are adding these fields now is to make the code generation consistent with the runtime library proposal under review in https://reviews.llvm.org/D14031.

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

Subscribers: cfe-commits, caomhin, jholewinski

Differential Revision: https://reviews.llvm.org/D28298

llvm-svn: 291124