GENERATED prop: Check CMP0118 policy and warn in certain situations

* Adding tests for CMP0118 being unset (aka set to `WARN`).
* Adjusting the (unrelated) RunCMake.CMP0026 test to set CMP0118 to
  `NEW`,
* Adjusting the (unrelated) RunCMake.Ninja test to set CMP0118 to `OLD`.
* Adjusting the (unrelated) RunCMake.FileAPI test to set CMP0118 to
  `OLD`.

Note:
Setting CMP0118 to `NEW` and modifying the `GENERATED` property with
`set_property` or `set_source_files_properties` will currently NOT set
that property because the implementation is still to come.

Source/cmSetSourceFilesPropertiesCommand.cxx

@@ -7,6 +7,7 @@
 
 #include <cm/string_view>
 #include <cmext/algorithm>
+#include <cmext/string_view>
 
 #include "cmExecutionStatus.h"
 #include "cmMakefile.h"
@@ -167,7 +168,13 @@ static bool RunCommandForScope(
     if (cmSourceFile* sf = mf->GetOrCreateSource(sfname)) {
       // loop through the props and set them
       for (auto k = propertyPairs.begin(); k != propertyPairs.end(); k += 2) {
-        sf->SetProperty(*k, (k + 1)->c_str());
+        // Special handling for GENERATED property?
+        if (*k == "GENERATED"_s) {
+          SetPropertyCommand::HandleAndValidateSourceFilePropertyGENERATED(
+            sf, *(k + 1));
+        } else {
+          sf->SetProperty(*k, (k + 1)->c_str());
+        }
       }
     }
   }