[TableGen] Use "parent class" terminology in TGParser. NFC. (#235106)
TGParser called the classes and multiclasses that a record or multiclass
inherits from "subclasses", for example in AddSubClass and
SubClassReference, although they are its parent classes. Rename them to
match the terminology of the TableGen grammar in ProgRef, e.g.
AddParentClass and ClassReference, and update the comments
accordingly, including the grammar comments.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
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Co-authored-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>