[DWARFv5] Support DW_FORM_strp in the .debug_line.dwo header.

As a side effect, the .debug_line section will be dumped in physical
order, rather than in the order that compile units refer to their
associated portions of the .debug_line section.  These are probably
always the same order anyway, and no tests noticed the difference.

Differential Revision: https://reviews.llvm.org/D39854

llvm-svn: 318839