[LLD] [COFF] Parse all /debug: options, like /opt: (#75178)

Most option handling is like it was before; the last /debug: option
takes effect.

However, the options /debug:dwarf or /debug:symtab don't reset all flags
into the specific behaviour they chose before - e.g. if an earlier
option enables writing a PDB, a later /debug:dwarf or /debug:symtab
doesn't disable that. This allows combining these options with options
for controlling PDB writing, for finetuning what is done.