[llvm] [CMake] Turn on LLVM_USE_SPLIT_DWARF by default for Linux Debug build (PR #80328)

[Jinsong Ji via llvm-commits]

jsji wrote:

> ideally we'd be able to detect when the "default" aren't suitable for the current platform/toolchain and error out at CMake configuration time with a very helpful message.

Thanks @dwblaikie and @joker-eph . Yes, I am happy to proactively work on attempts like https://github.com/llvm/llvm-project/pull/80500 if we know what to detect, just let me know. 
There might be situations that we don't know yet, then we can response to problem report in practice. 

https://github.com/llvm/llvm-project/pull/80328