[OpenMP] Add tests for mapping of chained 'containing' structs (#156703)
This PR adds several new tests for mapping of chained structures, i.e.
those resembling:
#pragma omp target map(tofrom: a->b->c)
These are currently XFAILed, although the first two tests actually work
with unified memory -- I'm not sure if it's possible to easily improve
the condition on the XFAILs in question to make them more accurate.
These cases are all fixed by the WIP PR
https://github.com/llvm/llvm-project/pull/153683.