[ELF] Optimize getInputSections. NFC

In the majority of cases (e.g. orphan sections), an OutputSection has at most
one InputSectionDescription (isd). By changing the return type to
ArrayRef<InputSection *> we can just reference the isd->sections. For
OutputSections with more than one InputSectionDescription we use a caller
provided SmallVector to copy the elements as before.

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D129111