[clang][NFC] Refactor enums that hold size of `Type` and `DeclContext` bit-fields (#70296)

This patch refactor said enums to hold total size of a bit-field, and not just non-inherited bits. This brings `Type` and `DeclContext` in line with `Comment` and `Stmt`. It also makes it unnecessary to list all transitive bases of a bit-field as unnamed bit-fields, which makes it more friendly towards debuggers.