[lld] [ELF] Implement -z dynamic-undefined-weak (PR #143831)

[Peter Smith via llvm-commits]

https://github.com/smithp35 approved this pull request.

Looks good to me. Could be worth mentioning in the description that this option is available in GNU ld.

I only realised when getting to the comment in the tests.

The description in the GNU manual https://sourceware.org/binutils/docs/ld/Options.html might be a good start for the ld.lld man page.

https://github.com/llvm/llvm-project/pull/143831