[ELF] Demote !isUsedInRegularObj lazy symbol

I think D79300 has fixed the D51892 (`__i686.get_pc_thunk.bx`) issue, so
we can bring back rL330869.
D79300 says `would error undefined symbol instead of the more relevant discarded section`
but it doesn't reproduce now.

This avoids a quirk in `isUndefWeak()`.

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D111365