Convert read_indirect_line_string to a method

Convert read_indirect_line_string to a method

This changes read_indirect_line_string to be a method on
dwarf2_per_objfile.  This makes it a bit simpler to share between
files.

gdb/ChangeLog
2020-03-26  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.h (struct dwarf2_per_objfile) <read_line_string>:
	Declare method.
	* dwarf2/read.c (read_attribute_value): Update.
	(dwarf2_per_objfile::read_line_string): Rename from
	read_indirect_line_string.
	(read_formatted_entries): Update.