[OpenMP] Add RTL function to externalization RAII

[jhuber6]

Summary

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.

[jdoerfert]

This was forgotten in D113602. LG

llvm/lib/Transforms/IPO/OpenMPOpt.cpp

bool runAttributor(bool IsModulePass) {
     ExternalizationRAII EndParallel(OMPInfoCache,
                                     OMPRTL___kmpc_kernel_end_parallel);
     ExternalizationRAII BarrierSPMD(OMPInfoCache,
                                     OMPRTL___kmpc_barrier_simple_spmd);
     ExternalizationRAII BarrierGeneric(OMPInfoCache,
                                        OMPRTL___kmpc_barrier_simple_generic);
     ExternalizationRAII ThreadId(OMPInfoCache,
                                  OMPRTL___kmpc_get_hardware_thread_id_in_block);
+    ExternalizationRAII WarpSize(OMPInfoCache, OMPRTL___kmpc_get_warp_size);
 
     registerAAs(IsModulePass);
 
     ChangeStatus Changed = A.run();
 
     LLVM_DEBUG(dbgs() << "[Attributor] Done with " << SCC.size()
                       << " functions, result: " << Changed << ".\n");