NullReferenceException when ClangSharpSourceToWinmd attempts to translate a function pointer

Given a function pointer (added in [C# 9](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-9.0/function-pointers)):

```csharp
namespace Test
{
public unsafe partial struct MyStruct
{
public delegate* unmanaged[Stdcall] Handler;
}
}
```

ClangSharpSourceToWinmd will raise a `NullReferenceException` in `ClangSharpSourceWimmdGenerator.GetTypeRefOrDef` as function pointer types have a `null` `ContainingNamespace`.

Start with ClangSharpSourceToWinmd and the ClangSharpSourceWimmdGenerator.GetTypeRefOrDef method named in the report. Reproduce the failure using the provided C# function-pointer example, then trace how its null ContainingNamespace is handled. Done means the example translates without a NullReferenceException.

Written by the indexing model from the issue text.