[ELF] - Teach LLD to report line numbers for data symbols.

This is PR34826.

Currently LLD is unable to report line number when reporting
duplicate declaration of some variable.

That happens because for extracting line information we always use
.debug_line section content which describes mapping from machine
instructions to source file locations, what does not help for
variables as does not describe them.

In this patch I am taking the approproate information about
variables locations from the .debug_info section.

Differential revision: https://reviews.llvm.org/D38721

llvm-svn: 317080