VS: Tell VS 16.4 not to verify SYMBOLIC custom command outputs

VS 16.4 introduces an additional check on `CustomBuild` rules that warns
if the outputs of the command are not created.  However, CMake supports
marking outputs with the `SYMBOLIC` property to indicate that they will
not actually be generated.  That property is used by Makefile and Ninja
generators but has not been needed by the VS generators before.  Teach
the VS generator to disable `VerifyInputsAndOutputsExist` in custom
build rules that have a symbolic output.

Fixes: #19737

Source/cmVisualStudio10TargetGenerator.cxx

@@ -1438,6 +1438,7 @@ void cmVisualStudio10TargetGenerator::WriteCustomRule(
     }
     // output files for custom command
     std::stringstream outputs;
+    bool symbolic = false;
     {
       const char* sep = "";
       for (std::string const& o : ccg.GetOutputs()) {
@@ -1445,6 +1446,12 @@ void cmVisualStudio10TargetGenerator::WriteCustomRule(
         ConvertToWindowsSlash(out);
         outputs << sep << out;
         sep = ";";
+        if (!symbolic) {
+          if (cmSourceFile* sf = this->Makefile->GetSource(
+                o, cmSourceFileLocationKind::Known)) {
+            symbolic = sf->GetPropertyAsBool("SYMBOLIC");
+          }
+        }
       }
     }
     if (this->ProjectType == csproj) {
@@ -1454,7 +1461,7 @@ void cmVisualStudio10TargetGenerator::WriteCustomRule(
                                   outputs.str(), comment);
     } else {
       this->WriteCustomRuleCpp(*spe2, c, script, additional_inputs.str(),
-                               outputs.str(), comment);
+                               outputs.str(), comment, symbolic);
     }
   }
 }
@@ -1462,7 +1469,7 @@ void cmVisualStudio10TargetGenerator::WriteCustomRule(
 void cmVisualStudio10TargetGenerator::WriteCustomRuleCpp(
   Elem& e2, std::string const& config, std::string const& script,
   std::string const& additional_inputs, std::string const& outputs,
-  std::string const& comment)
+  std::string const& comment, bool symbolic)
 {
   const std::string cond = this->CalcCondition(config);
   e2.WritePlatformConfigTag("Message", cond, comment);
@@ -1474,6 +1481,13 @@ void cmVisualStudio10TargetGenerator::WriteCustomRuleCpp(
     // VS >= 11 let us turn off linking of custom command outputs.
     e2.WritePlatformConfigTag("LinkObjects", cond, "false");
   }
+  if (symbolic &&
+      this->LocalGenerator->GetVersion() >=
+        cmGlobalVisualStudioGenerator::VS16) {
+    // VS >= 16.4 warn if outputs are not created, but one of our
+    // outputs is marked SYMBOLIC and not expected to be created.
+    e2.WritePlatformConfigTag("VerifyInputsAndOutputsExist", cond, "false");
+  }
 }
 
 void cmVisualStudio10TargetGenerator::WriteCustomRuleCSharp(

Source/cmVisualStudio10TargetGenerator.h

@@ -142,7 +142,7 @@ private:
                           std::string const& script,
                           std::string const& additional_inputs,
                           std::string const& outputs,
-                          std::string const& comment);
+                          std::string const& comment, bool symbolic);
   void WriteCustomRuleCSharp(Elem& e0, std::string const& config,
                              std::string const& commandName,
                              std::string const& script,