[libomptarget-nvptx] Fix ancestor_thread_num and team_size (non-SPMD)

According to OpenMP 4.5, p250:12-14:

    If the requested nest level is outside the range of 0 and the
    nest level of the current thread, as returned by the omp_get_level
    routine, the routine returns -1.

The SPMD code path will need a similar fix.

Differential Revision: https://reviews.llvm.org/D51787

llvm-svn: 343401