Fix v22 regen regressions: vftable dtor index, --with-base, Dispose, namespace qualifier

[tannergooding]

Fourth batch of fixes for regressions found regenerating terrafx.interop.windows against
ClangSharp v22.1.8. Each fix is a separate commit and has its own repro; the two SERIOUS
correctness issues are G1 and G2.

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G1 -- Microsoft vftable index for virtual destructors (native)

clang 22's MicrosoftVTableContext reports Index == 0 for a virtual destructor's vftable
location regardless of its declaration position, so the destructor collides with whatever
occupies slot 0 whenever it is not declared first. In terrafx this moved CHttpModule's
trailing ~CHttpModule from [VtblIndex(30)] (after the 30 On* notification methods) to
[VtblIndex(0)], where it aliases OnBeginRequest -- wrong vtable dispatch.

The vftable layout itself is ordered correctly, so getVtblIdx now recovers the destructor's
true slot by counting the function-pointer components that precede the deleting-destructor
component (non-slot components such as offsets are skipped so the count stays in the same space
as MethodVFTableLocation::Index).

Validated by building libClangSharp against LLVM 22.1.8 locally and exercising it through both
the interop layer and the generator: trailing, middle, leading, overloaded, and pure-virtual
destructor placements all resolve to the correct slot, and the full generator suite still passes
with the rebuilt native (no existing baseline changes). The checked-in golden tests load the
pinned prebuilt libClangSharp package (which still has the bug), so the managed pin bump, any
baseline regeneration, and a destructor-slot regression test are a follow-up once 22.1.8.3 is
published -- see the version-bump commit.

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G2 -- --with-base for injecting additional base types

terrafx hand-maintains IUnknown.Interface : INativeGuid, which the regen drops (the root COM
interface is the one interface that doesn't otherwise derive it). Rather than special-case it,
this adds a --with-base / -wb option that lets a generation inject additional base type(s)
onto a type, keyed the same way as other remaps (Type=Base). The extra bases land on the
marker Interface for vtbl types when marker interfaces are generated, and on the struct
itself otherwise.

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G3 -- empty Dispose() for structs whose destructor frees members

A struct whose C++ destructor delete[]s pointer members (e.g. GpPathData) emitted a
Dispose() whose bodies were bare empty statements -- a silent leak. VisitStmt now emits the
cxx_delete for delete/delete[] expressions in destructor bodies.

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G4 -- doubled namespace qualifier in NativeTypeName

GetSource's already-qualified Windows::Foundation::IPropertyValue ** was re-prefixed to
ABI::Windows::Foundation::Windows::Foundation::IPropertyValue **. The qualifier is now deduped
against the existing source spelling. Cosmetic (the C# type was already correct).

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Out of scope / follow-up: bumping the managed libClangSharp pin to 22.1.8.3, regenerating any
affected baselines, and adding the G1 destructor-slot golden test, all of which depend on the
22.1.8.3 native package being published first.