[new] 新增 MsvcStdextWorkaround.cs 解决2020 vs下stdext编译错误的问题

Editor/BuildProcessors/MsvcStdextWorkaround.cs

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+#if UNITY_EDITOR
+using System;
+using UnityEditor.Build;
+using UnityEditor.Build.Reporting;
+
+public class MsvcStdextWorkaround : IPreprocessBuildWithReport
+{
+    const string kWorkaroundFlag = "/D_SILENCE_STDEXT_HASH_DEPRECATION_WARNINGS";
+
+    public int callbackOrder => 0;
+
+    public void OnPreprocessBuild(BuildReport report)
+    {
+        var clEnv = Environment.GetEnvironmentVariable("_CL_");
+
+        if (string.IsNullOrEmpty(clEnv))
+        {
+            Environment.SetEnvironmentVariable("_CL_", kWorkaroundFlag);
+        }
+        else if (!clEnv.Contains(kWorkaroundFlag))
+        {
+            clEnv += " " + kWorkaroundFlag;
+            Environment.SetEnvironmentVariable("_CL_", clEnv);
+        }
+    }
+}
+
+#endif // UNITY_EDITOR

Editor/BuildProcessors/MsvcStdextWorkaround.cs.meta

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