Create dwarf2/attribute.[ch]

Create dwarf2/attribute.[ch]

This moves the attribute-related code out of dwarf2read.c and into the
new files dwarf2/attribute.[ch].

gdb/ChangeLog
2020-02-08  Tom Tromey  <tom@tromey.com>

	* dwarf2read.c (struct attribute, DW_STRING)
	(DW_STRING_IS_CANONICAL, DW_UNSND, DW_BLOCK, DW_SND, DW_ADDR)
	(DW_SIGNATURE, struct dwarf_block, attr_value_as_address)
	(attr_form_is_block, attr_form_is_section_offset)
	(attr_form_is_constant, attr_form_is_ref): Move.
	* dwarf2/attribute.h: New file.
	* dwarf2/attribute.c: New file, from dwarf2read.c.
	* Makefile.in (COMMON_SFILES): Add dwarf2/attribute.c.

Change-Id: I1ea4c146256a1b9e38b66f1c605d782a14eeded7