VS: Do not reference output assemblies if not possible for CSharp target

Since commit v3.9.0-rc4~4^2 (Vs: allow CSharp targets to be linked to
CXX targets, 2017-06-20) CSharp targets get `ProjectReference` entries
to their dependencies.  This causes VS to also reference the
dependency's output assembly by default, which is incorrect for
non-managed targets.

Fix this by setting `ReferenceOutputAssembly` to `false` for targets
that can't provide output assemblies.  Unmanaged C++ targets (shared
libs & executables) can still be referenced and a warning will be shown
in the IDE but the build will not break anymore.

Fixes: #17172

Source/cmVisualStudio10TargetGenerator.cxx

@@ -3502,6 +3502,13 @@ void cmVisualStudio10TargetGenerator::WriteProjectReferences()
     (*this->BuildFileStream) << "</Project>\n";
     this->WriteString("<Name>", 3);
     (*this->BuildFileStream) << name << "</Name>\n";
+    if (csproj == this->ProjectType) {
+      if (!static_cast<cmGlobalVisualStudioGenerator*>(this->GlobalGenerator)
+             ->TargetCanBeReferenced(dt)) {
+        this->WriteString(
+          "<ReferenceOutputAssembly>false</ReferenceOutputAssembly>\n", 3);
+      }
+    }
     this->WriteString("</ProjectReference>\n", 2);
   }
   this->WriteString("</ItemGroup>\n", 1);