[libcxxabi] [libunwind] [libcxxabi][libunwind] Support for using LLVM libc (PR #101688)

[Louis Dionne via cfe-commits]

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ldionne wrote:

For now, can't you include `libcxx/cmake/Modules/HandleLibC.cmake` instead?

https://github.com/llvm/llvm-project/pull/101688