[OpenMP] Add RTL function to externalization RAII

This patch adds the `__kmpc_get_warp_size` OpenMP RTL function to the
externalization RAII struct. This was getting optimized out and then
being replaced with an undefined value once added back in, causing bugs
for complex reductions.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D114802