Revert "[flang][OpenMP] Fix namespace nesting after PR144960"

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

This reverts commit 925dbc7988.

flang/lib/Lower/OpenMP/Atomic.h

@@ -12,16 +12,16 @@ namespace Fortran {
 namespace lower {
 class AbstractConverter;
 class SymMap;
-
-namespace pft {
-struct Evaluation;
-}
 } // namespace lower
 
 namespace parser {
 struct OpenMPAtomicConstruct;
 }
 
+namespace pft {
+struct Evaluation;
+}
+
 namespace semantics {
 class SemanticsContext;
 }