[NFC][OpenMP][Offloading] Replaced explicit parallel level computation with...

[NFC][OpenMP][Offloading] Replaced explicit parallel level computation with function `__kmpc_parallel_level`

There are two places in current deviceRTLs where it computes parallel level explicitly,
which is basically the functionality of `__kmpc_parallel_level`. Starting from
D105787, we plan to introduce a series of function call folding based on information
that can be deducted during compilation time. Computation of parallel level is
the next target. This patch makes steps for the optimization.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D105955