[lld] [LLD] [COFF] Rewrite handling of the /debug: option. NFC. (PR #75175)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

This goes on top of #75172; only the latter commit should be reviewed in this PR. The overall direction of the larger patchset that this belongs to, was presented/discussed in https://github.com/llvm/llvm-project/pull/74820.

https://github.com/llvm/llvm-project/pull/75175