[ELF] - Teach LLD to use information from .debug_str for error reporting.

Recently we teached LLD to report line numbers for duplicate variables
definitions, though currently LLD is unable to do that for case when
strings are not built in .debug_info, but stored in .debug_str instead.
That is because out LLDDwarfObj does not handle .debug_str yet. 
Patch fixes that.

Differential revision: https://reviews.llvm.org/D39542

llvm-svn: 317305