Micro-optimize abbrev reading and storage

Micro-optimize abbrev reading and storage

Currently, and abbrev_info points to a separately allocated array of
attr_abbrev objects.  This array is constructed in a temporary vector,
then copied to the abbrev table's obstack.

This patch changes abbrev_info to use the struct hack to store the
objects directly, and changes abbrev_table::read to avoid an extra
copy when allocating, using the "growing objects" capability of
obstacks.

This saves a bit of space, and also perhaps a little time.

2021-03-06  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.c (read_attribute): Make 'abbrev' const.
	* dwarf2/abbrev.c (abbrev_table::alloc_abbrev): Remove.
	(abbrev_table::read): Update.
	* dwarf2/abbrev.h (struct attr_abbrev): Move earlier.
	(struct abbrev_info): Reformat.
	<attrs>: Now an array.
	(struct abbrev_table) <alloc_abbrev>: Remove.