[libc++abi] NFC: avoid a -Wunused-parameter warning

Summary:
A -Wunused-parameter warning was introduced by patch rG7f0244afa828 [libc++abi] NFC: adding a new parameter base to functions for calculating… (authored by xingxue). The unused parameter base will be used in a follow-on patch D101298. This patch is to avoid the warning before D101298 is landed.

Reviewers: ldionne, sfertile, compnerd, libc++abi

Reviewed by: ldionne

Differential Revision: https://reviews.llvm.org/D104235