Add dwz.c and dwz_file::read_string

Add dwz.c and dwz_file::read_string

This changes read_indirect_string_from_dwz to be a method on the
dwz_file, and adds a new dwarf2/dwz.c file.

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

	* dwarf2/read.c (read_attribute_value): Update.
	(read_indirect_string_from_dwz): Move to dwz.c; change into
	method.
	(dwarf_decode_macro_bytes): Update.
	* dwarf2/dwz.h (struct dwz_file) <read_string>: Declare method.
	* dwarf2/dwz.c: New file.
	* Makefile.in (COMMON_SFILES): Add dwz.c.