Allow nested types inside of the <Module> type

- This isn't disallowed by spec, although ilasm and ildasm cannot handle these cases
- Simply skip adding the type to the available class hash.

Backport of bugfix portion of PR #111435
Fixes #111164 in .NET 9

src/coreclr/vm/clsload.cpp

@@ -3482,6 +3482,12 @@ VOID ClassLoader::AddAvailableClassHaveLock(
     if (SUCCEEDED(pMDImport->GetNestedClassProps(classdef, &enclosing))) {
         // nested type
 
+        if (enclosing == COR_GLOBAL_PARENT_TOKEN)
+        {
+            // Types nested in the <module> class can't be found by lookup.
+            return;
+        }
+
         COUNT_T classEntryIndex = RidFromToken(enclosing) - 1;
         _ASSERTE(RidFromToken(enclosing) < RidFromToken(classdef));
         if (classEntries->GetCount() > classEntryIndex)