Revert "[flang][OpenMP] Move lowering of ATOMIC to separate file, NFC (#146067)"

This reverts commit 302ed97.

Author: kparzysz

flang/lib/Lower/CMakeLists.txt

@@ -23,7 +23,6 @@ add_flang_library(FortranLower
   LoweringOptions.cpp
   Mangler.cpp
   OpenACC.cpp
-  OpenMP/Atomic.cpp
   OpenMP/ClauseProcessor.cpp
   OpenMP/Clauses.cpp
   OpenMP/DataSharingProcessor.cpp