[CodeGen] Silence unused variable warning in Release builds.

llvm-svn: 296034

clang/lib/CodeGen/CGVTables.cpp

@@ -284,7 +284,9 @@ void CodeGenFunction::EmitCallAndReturnForThunk(llvm::Constant *CalleePtr,
   if (isa<CXXDestructorDecl>(MD))
     CGM.getCXXABI().adjustCallArgsForDestructorThunk(*this, CurGD, CallArgs);
 
+#ifndef NDEBUG
   unsigned PrefixArgs = CallArgs.size() - 1;
+#endif
   // Add the rest of the arguments.
   for (const ParmVarDecl *PD : MD->parameters())
     EmitDelegateCallArg(CallArgs, PD, SourceLocation());