[NFC][Clang] Move function implementation of `OpenMPAtomicUpdateChecker` into anonymous namespace

[tianshilei1992]

Summary

Just to keep code consistent as OpenMPAtomicUpdateChecker is defined
in anonymous namespace.

[ABataev]

LG

clang/lib/Sema/SemaOpenMP.cpp

public:
   /// true if the source expression is a postfix unary operation, false
   /// if it is a prefix unary operation.
   bool isPostfixUpdate() const { return IsPostfixUpdate; }
 
 private:
   bool checkBinaryOperation(BinaryOperator *AtomicBinOp, unsigned DiagId = 0,
                             unsigned NoteId = 0);
 };
-} // namespace
 
 bool OpenMPAtomicUpdateChecker::checkBinaryOperation(
     BinaryOperator *AtomicBinOp, unsigned DiagId, unsigned NoteId) {
   ExprAnalysisErrorCode ErrorFound = NoError;
   SourceLocation ErrorLoc, NoteLoc;
   SourceRange ErrorRange, NoteRange;
   // Allowed constructs are:
   //  x = x binop expr;
if (ErrorFound == NoError && E && X) {
     Update = SemaRef.PerformImplicitConversion(Update.get(), X->getType(),
                                                Sema::AA_Casting);
     if (Update.isInvalid())
       return true;
     UpdateExpr = Update.get();
   }
   return ErrorFound != NoError;
 }
+} // namespace
 
 StmtResult Sema::ActOnOpenMPAtomicDirective(ArrayRef<OMPClause *> Clauses,
                                             Stmt *AStmt,
                                             SourceLocation StartLoc,
                                             SourceLocation EndLoc) {
   // Register location of the first atomic directive.
   DSAStack->addAtomicDirectiveLoc(StartLoc);
   if (!AStmt)