[AbstractAttributor] Fold __kmpc_parallel_level if possible

Similar to D105787, this patch tries to fold `__kmpc_parallel_level` if possible.
Note that `__kmpc_parallel_level` doesn't take activeness into consideration,
based on current `deviceRTLs`, its return value can be such as 0, 1, 2, instead
of 0, 129, 130, etc. that also indicate activeness.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D106154