VS: Fix crash on CSharp sources in a custom target

The target generator does not compute ClOptions for custom targets,
so we should not use them either.

Fixes: #18377, #18485

Source/cmVisualStudio10TargetGenerator.cxx

@@ -1130,6 +1130,10 @@ void cmVisualStudio10TargetGenerator::WriteMSToolConfigurationValues(
 void cmVisualStudio10TargetGenerator::WriteMSToolConfigurationValuesManaged(
   Elem& e1, std::string const& config)
 {
+  if (this->GeneratorTarget->GetType() > cmStateEnums::OBJECT_LIBRARY) {
+    return;
+  }
+
   cmGlobalVisualStudio10Generator* gg = this->GlobalGenerator;
 
   Options& o = *(this->ClOptions[config]);

Tests/CSharpOnly/CMakeLists.txt

@@ -8,3 +8,6 @@ add_library(lib2 SHARED lib2.cs)
 add_executable(CSharpOnly csharponly.cs)
 
 target_link_libraries(CSharpOnly lib1 lib2)
+
+add_custom_target(CSharpCustom SOURCES empty.cs)
+add_custom_target(custom.cs DEPENDS empty.txt)