[OpenMP] Define omp_is_initial_device() variants in omp.h

omp_is_initial_device() is marked as a built-in function in the current
compiler, and user code guarded by this call may be optimized away,
resulting in undesired behavior in some cases. This patch provides a
possible fix for such cases by defining the routine as a variant
function and removing it from builtin list.

Differential Revision: https://reviews.llvm.org/D99447