[OPENMP] Fix PR47999: correctly map implicit firstprivates in outer tasks.

If the variable is implicitly firstprivatized in the inner task-based
region, it also must be firstprivatized in outer task-based regions.
Previously firstprivates were captured in tasks but later it was
optimized to reduce the memory usage. But still need to mark such
variables as implicit firstprivate in outer tasks.

Differential Revision: https://reviews.llvm.org/D91627