[DWARFv5] Support DW_FORM_line_strp in llvm-dwarfdump.

This form is like DW_FORM_strp, but points to .debug_line_str instead
of .debug_str as the string section.  It's intended to be used from
the line-table header, and allows string-pooling of directory and
filenames across compilation units.

Differential Revision: https://reviews.llvm.org/D42553

llvm-svn: 323476