Introduce new OpenMP 5.0 depend object type.

The implementation should be done by compiler, user can only declare
objects of this type and use them in OpenMP directives.

Differential Revision: https://reviews.llvm.org/D61860


git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@360774 91177308-0d34-0410-b5e6-96231b3b80d8