Add dwarf2_section_info::read_string method

Add dwarf2_section_info::read_string method

This moves a string-reading function to be a method on
dwarf2_section_info, and then updates the users.

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

	* dwarf2/section.h (struct dwarf2_section_info) <read_string>: New
	method.
	* dwarf2/section.c: New method.  From
	read_indirect_string_at_offset_from.
	* dwarf2/read.c (mapped_debug_names::namei_to_name): Update.
	(read_indirect_string_at_offset_from): Move to section.c.
	(read_indirect_string_at_offset): Rewrite.
	(read_indirect_line_string_at_offset): Remove.
	(read_indirect_string, read_indirect_line_string)
	(dwarf_decode_macro_bytes): Update.