[DWARF] Change pubnames to use DWARFSection instead of StringRef

Summary: The debug_info_offset values in .debug_{,gnu_}pub{name,types} may be relocated. Change it to DWARFSection so that we can get relocated values.

Reviewers: ruiu, dblaikie, grimar, JDevlieghere

Reviewed By: JDevlieghere

Subscribers: aprantl, JDevlieghere, llvm-commits

Differential Revision: https://reviews.llvm.org/D54375

llvm-svn: 346615