Use htab_up in abbrev_table

Use htab_up in abbrev_table

This changes abbrev_table to use an htab_up rather than an ad hoc,
bucket-based hash table.

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

	* dwarf2/abbrev.c (abbrev_table): Move constructor from header.
	Rewrite.
	(abbrev_table::add_abbrev, abbrev_table::lookup_abbrev): Rewrite.
	* dwarf2/abbrev.h (struct abbrev_info) <next>: Remove.
	(abbrev_table::abbrev_table): No longer inline.
	(ABBREV_HASH_SIZE): Remove.
	(abbrev_table::m_abbrevs): Now an htab_up.

Change-Id: Icbaa8e49501f9c43218d6a81a7e8c4d3a77d65dc