[OpenMP] Make the new offloading driver the default

Previously an opt-in flag `-fopenmp-new-driver` was used to enable the
new offloading driver. After passing tests for a few months it should be
sufficiently mature to flip the switch and make it the default. The new
offloading driver is now enabled if there is OpenMP and OpenMP
offloading present and the new `-fno-openmp-new-driver` is not present.

The new offloading driver has three main benefits over the old method:
- Static library support
- Device-side LTO
- Unified clang driver stages

Depends on D122683

Differential Revision: https://reviews.llvm.org/D122831