[flang][OpenMP] Fix namespace nesting after PR144960

Newly introduced Atomic.cpp fails to compile on its own, but somehow
compiles fine in the build. Maybe it's because PCH, but it needs to be
fixed nevertheless.

Author: kparzysz

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;
 }