[libunwind] Support the new libc++ test format

Reviewers: ldionne, #libunwind, mstorsjo

Reviewed By: ldionne, #libunwind, mstorsjo

Subscribers: mstorsjo, dexonsmith, llvm-commits, libcxx-commits

Tags: #llvm, #libunwind

Differential Revision: https://reviews.llvm.org/D77501