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

[Petr Hosek via cfe-commits]

petrhosek wrote:

> @petrhosek @ldionne would it be better detect libunwind in LLVM_ENABLE_RUNTIMES?
> 
> Asking because we have similar thing in compiler-rt, e.g. for lld, and I am not sure which approach is better: There are trade offs:
> 
> 1. checking LLVM_ENABLE_RUNTIMES simplify end user experience
> 2. forcing default  ON simplifies cmake files and avoids unexpected use of system lib

That would be my preference, specifically I'd set the default for `LIBCXXABI_USE_LLVM_UNWINDER` based on whether `libunwind` is present in `LLVM_ENABLE_RUNTIMES` (but still let user override it).

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