lld: Avoid decompressing unused debug sections

[pcc]

When looking at a perf profile of a link with objects with compressed debug info and -S in ldflags, I noticed that a substantial amount of time was spent in MergeInputSection::splitIntoPieces -> InputSectionBase::decompress. This seems to be because the code that implements section stripping operates on ctx.inputSections while splitSections operates on ctx.objectFiles.