[clang] NFC: rename SK_CastDerivedToBaseRValue to SK_CastDerivedToBasePRValue

This is a follow up to the "rvalue-to-prvalue" rename at D103720.

Signed-off-by: Matheus Izvekov <mizvekov@gmail.com>

Depends on D103720

Reviewed By: rsmith

Differential Revision: https://reviews.llvm.org/D103933