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

Reinstate commits e5559ca and 925dbc7 with changes that avoid the
reported failures in Windows builds.

Ref: llvm/llvm-project#144960

Author: kparzysz

flang/lib/Lower/CMakeLists.txt

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