Degrade gracefully on C++ pointer-to-member types

[tannergooding]

C++ pointer-to-member types (int MyClass::*, void (MyClass::*)()) were unsupported: the type-class switch never mapped CX_TypeClass_MemberPointer, so GetTypeName fell through to the raw Clang spelling (int MyClass::*, which isn't valid C#) and GetTypeSize hard-errored with Unsupported type: 'CX_TypeClass_MemberPointer'. The containing struct then couldn't be laid out.

These have no direct C# equivalent, so this degrades gracefully rather than erroring: GetTypeName emits an opaque void* and preserves the original spelling via [NativeTypeName(...)] plus a single warning, and GetTypeSize treats member pointers as pointer-sized (added to the existing FunctionType or PointerType or ReferenceType branch).

public unsafe partial struct MyStruct
{
    [NativeTypeName("int MyClass::*")]
    public void* field;
}

---

Caveat, called out in the warning: a member-function pointer can be wider than a data pointer under both the Itanium and MSVC ABIs, so void* may under-size those. Getting the exact width right is ABI-specific and left as a separate follow-up; this change is about not hard-erroring and keeping the struct usable.

Adds a golden regression test (MemberPointerDeclarationTest) covering data- and function-member pointers, with per-OS expected diagnostics (MSVC additionally emits an MSInheritance attribute warning; Itanium does not).

Fixes #511