[CIR] Introduce cir::RecordKind::Class (#142690)
When cir::RecordType was initially upstreamed, we decided that there was
no reason to distinguish between RecordKind::Class and
RecordKind::Struct since they are semantically equivalent. I think this
was a mistake because classic codegen does preserve the distinction
(which is visible in the AST) and preserving the distinction in CIR will
aid the possibility of eventually using the classic codegen lit tests
with CIR.
This change introduces RecordKind::Class, which is already present in
the incubator implementation.
