[ELF] Change GnuPub{Names,Types}Section from StringRef to LLDDWARFSection

Summary:
The debug_info_offset value may be relocated.

This is lld side change of D54375.

Reviewers: ruiu, dblaikie, grimar, espindola

Subscribers: emaste, arichardson, JDevlieghere, llvm-commits

Differential Revision: https://reviews.llvm.org/D54376

llvm-svn: 346616