[OpenMP] Fix always,from and delete for data absent at exit

Without this patch, there's a runtime error for those map types at
exit from an "omp target data" or at "omp target exit data", but the
spec says the list item should be ignored.

This patch tests that fix in data_absent_at_exit.c, and it also
improves other testing for data that is not fully present at exit.

Reviewed By: grokos, RaviNarayanaswamy

Differential Revision: https://reviews.llvm.org/D96999