Minor cleanups in abbrev_table

Minor cleanups in abbrev_table

This cleans up the DWARF abbrev_table API a bit, primarily by making
various methods and members private.

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

	* dwarf2/read.c (read_cutu_die_from_dwo): Update.
	(cutu_reader): Update.
	(build_type_psymtabs_1): Update.
	* dwarf2/abbrev.c (abbrev_table::read): Rename.
	(abbrev_table::alloc_abbrev): Update.
	* dwarf2/abbrev.h (abbrev_table_up): Move earlier.
	(abbrev_table::read): New static method, renamed from
	abbrev_table_read_table.
	(abbrev_table::alloc_abbrev)
	(abbrev_table::add_abbrev): Now private.
	(abbrev_table::abbrev_table): Now private.
	(abbrev_table::m_abbrev_obstack): Now private.  Rename.

Change-Id: I320dca83b799f672909ae66f73b7aca266adbaf9