[LLVMABI][NFC] Align base class handling with Clang's AST (#218545)
The LLVM ABI library's RecordType was omitting direct virtual base
classes from its base class vector, which was a divergence from the
representation in the Clang AST. Clang's AST includes direct virtual
bases in both the collection of base classes and the collection of
virtual bases.
Aligning the handling between the Clang AST and the LLVM ABI RecordType
will simplifying porting of ABI classification for future targets.
Assisted-by: Cursor / various models