[DEBUGINFO] Add flag for DWARF2 or less to use sections as references.

Summary:
Some targets does not support labels inside debug sections, but support
references in form `section +|- offset`. Patch adds initial support
for this. Also, this patch disables emission of all additional debug
  sections that may have labels inside of it (like pub sections and
  string tables).

Reviewers: probinson, echristo

Subscribers: JDevlieghere, llvm-commits

Differential Revision: https://reviews.llvm.org/D43627

llvm-svn: 326328