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

Reinstate commits e5559ca and 925dbc7. Fix the issues with compilation
hangs by including DenseMapInfo specialization where the corresponding
DenseMap was defined.

Ref: #144960

Author: kparzysz

flang/include/flang/Lower/AbstractConverter.h

@@ -15,6 +15,7 @@
 
 #include "flang/Lower/LoweringOptions.h"
 #include "flang/Lower/PFTDefs.h"
+#include "flang/Lower/Support/Utils.h"
 #include "flang/Optimizer/Builder/BoxValue.h"
 #include "flang/Optimizer/Dialect/FIRAttr.h"
 #include "flang/Semantics/symbol.h"
@@ -23,6 +24,7 @@
 #include "mlir/IR/BuiltinOps.h"
 #include "mlir/IR/Operation.h"
 #include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/DenseMap.h"
 
 namespace mlir {
 class SymbolTable;

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