[clang] [llvm] [clang][Mach-O] Add a flag to force UNWIND_*_MODE_DWARF compact unwind info (PR #204005)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> The new flag is spelled -femit-dwarf-unwind=dwarf-only.

To be pedantic: this is a new option value. We use `flag` to refer to boolean options.

https://github.com/llvm/llvm-project/pull/204005