[ELF] Fixing an issue in Elf_Note_Impl::getDescAsStringRef

Summary:
Fix in getDescAsStringRef to not use a reference to a
temporary type and added a testcase.

Reviewers: arsenm, saiislam, scott.linder

Reviewed By: scott.linder

Subscribers: wdng, mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81653