Revert "[flang][OpenMP] Move lowering of ATOMIC to separate file, NFC (llvm#144960)"

PR#144960 broke check-flang tests on Windows (x64/ARM64).

This reverts commit e5559ca.

Author: omjavaid

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