[lld] [ELF] Allow KEEP within OVERLAY (PR #130661)

[Nathan Chancellor via llvm-commits]

nathanchance wrote:

> Should there be tests that we don't support the things that GNU says are disallowed in overlays?

I think that `err1.t` in `lld/test/ELF/linkerscript/overlay.test` already does that. As far as I can tell, the only thing this PR is allowing is `KEEP()` based on my reading and understanding of the differences between the current `readOverlaySectionDescription()` and `readInputSectionDescription()`.

https://github.com/llvm/llvm-project/pull/130661