[openmp] [llvm] [libcxx] [mlir] [libcxxabi] [lld] [compiler-rt] [clang] [runtimes] Use LLVM libunwind from libc++abi by default (PR #77687)

[Louis Dionne via cfe-commits]

ldionne wrote:

That means that based on whether `LLVM_ENABLE_RUNTIMES=libunwind` happens to be set or not, you'd use a fundamentally different unwinding library which implies a different ABI. It's convenient, but isn't that too subtle?

https://github.com/llvm/llvm-project/pull/77687