Don't emit duplicate members for structs with multiple vtbl bases

[tannergooding]

Fixes #592.

A struct deriving from two or more bases that each carry a virtual table flattens every base into the single lpVtbl. Members from different bases that map to the same C# name and signature -- most notably each base's virtual destructor becoming Dispose -- were emitted more than once, producing a CS0111 compile error.

Deduplicate the flattened vtbl members by their emitted name and canonical parameter types so the output at least compiles. This is correct for the destructor-unifying case: ~Foo/~Bar are both overridden by the implicit Baz::~Baz, and the primary vtable slot destroys the complete object, so a single Dispose() via lpVtbl[0] is right. Legitimate cross-base overloads with differing signatures are preserved; only true CS0111/CS0102 collisions are dropped. The fix covers both the function-pointer helper path and the explicit-vtbls path.

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Also ensure the 'multiple virtual bases' warning fires even when the derived type introduces its own virtual method. Previously HasVtbl only counted the indirect vtables inside the !hasVtbl branch, so the non-unifying case -- distinct secondary-base methods flattened onto the primary lpVtbl (e.g. Bar::BarMethod dispatched through lpVtbl[0], colliding with Foo::FooMethod) -- was emitted with incorrect dispatch and no diagnostic at all. The count is now computed unconditionally; hasBaseVtbl assignment is unchanged, so generated code is byte-identical and only the diagnostic differs.

The bindings remain incomplete for multiple virtual bases (the warning still reports this), since correctly modeling secondary-base dispatch requires a distinct vtable pointer per base subobject. That redesign is deferred to a follow-up.

Adds three golden regression tests (fnptr dedupe, explicit-vtbl dedupe, and the non-unifying warned case).