Revert "[flang][OpenMP] Move lowering of ATOMIC to separate file, NFC" (llvm#146091)

Reverts llvm#146067

This still causes timeouts, e.g.

https://lab.llvm.org/buildbot/#/builders/207/builds/3023/steps/7/logs/stdio

Author: kparzysz

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