[DWARF] Regularize dumping strings from line tables.

The major visible difference here is that in line-table dumps,
directory and file names are wrapped in double-quotes; previously,
directory names got single quotes and file names were not quoted at
all.

The improvement in this patch is that when a DWARF v5 line table
header has indirect strings, in a verbose dump these will all have
their section[offset] printed as well as the name itself.  This
matches the format used for dumping strings in the .debug_info
section.

Differential Revision: https://reviews.llvm.org/D42802

llvm-svn: 324270