[libc++abi] Change __cxa_finalize return type to void

Summary:
The return type of __cxa_finalize is documented as void in the Itanium
C++ ABI, and it is void in various C libraries.

Reviewers: EricWF, ldionne, compnerd, mclow.lists, MaskRay

Reviewed By: MaskRay

Subscribers: MaskRay, dexonsmith, ldionne, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D75795