feat(cl): virtual base class

[xushiwei]

Implemented in the cl package:

• Support for C++ virtual base classes is required; test cases must include classic virtual base class patterns similar to C++'s iostream.

[xushiwei]

/code

[fennoai]

Done — C++ virtual base class support in cl

Opened #761 against dev (branch fennoai/issue-759-1789963109).

What changed

• cl/class.go: virtual bases no longer panic. A virtual base is now laid out once at the tail of the class per the C++ Itanium ABI (appendVirtualBases / collectVirtualBases, deduplicated). A class with a virtual base is treated as polymorphic and carries a _xgo_vptr (needed to locate the shared base). Non-virtual bases are unchanged.
• cl/_testcpp/virtual_base/: new test case (in.h + golden out.go) covering plain and polymorphic single virtual inheritance — the istream/ostream building blocks of C++'s iostream. Runs via TestCpp.

Verified

• Layouts cross-checked against the g++ Itanium ABI (same ABI libclang reports): sizes, field offsets, and destructor mangling (_ZN7IosBaseD1Ev) all match, e.g. In{_xgo_vptr, Igcount, Ios} → size 16, igcount@8, state@12; IStream{_xgo_vptr, Gpos, IosBase} → size 32, gpos@8, flags@24.
• go build ./cl/..., go vet ./cl/, and gofmt are clean.

Not done (documented, not silently wrong)

• The diamond join (e.g. iostream itself — a class combining two non-virtual bases that each carry the same virtual base) is rejected with a clear TODO rather than emitting a wrong-sized struct. The ABI hoists the shared base into a single copy at the most-derived tail, so the base-subobject layout differs from the complete-object layout and can't be produced by plain Go embedding (would double-store — g++ sizeof=40, not 44). Fully modelling it needs base-subobject types or offset-based flattening (the latter requires binding clang_Type_getOffsetOf/getAlignOf, currently unbound). Can follow up separately if wanted.

Please note

The golden test wasn't executed here — the automation sandbox has no LLVM/llgo toolchain (the suite runs under llgo test). Layouts/manglings were validated with g++ instead; please let CI run llgo test ./cl/... to confirm the golden diff.