[flang][OpenMP] Move lowering of ATOMIC to separate file, NFC#146225

kparzysz merged 2 commits intollvm:mainllvm/llvm-project:mainfrom

kparzysz:users/kparzysz/atomic-sema-splitkparzysz/llvm-project:users/kparzysz/atomic-sema-splitCopy head branch name to clipboard