[flang][OpenMP] Move lowering of ATOMIC to separate file, NFC#146067

kparzysz merged 1 commit intollvm:mainllvm/llvm-project:mainfrom

kparzysz:users/kparzysz/atomic-split-rekparzysz/llvm-project:users/kparzysz/atomic-split-reCopy head branch name to clipboard