Create dwarf2/abbrev.[ch]

Create dwarf2/abbrev.[ch]

This moves the abbrev table code out of dwarf2read.c and into new
files dwarf2/abbrev.[ch].

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

	* dwarf2read.c (abbrev_table_up, struct abbrev_info)
	(struct attr_abbrev, ABBREV_HASH_SIZE, struct abbrev_table):
	Move.
	(read_cutu_die_from_dwo, build_type_psymtabs_1): Update.
	(abbrev_table::alloc_abbrev, abbrev_table::add_abbrev)
	(abbrev_table::lookup_abbrev, abbrev_table_read_table): Move to
	abbrev.c.
	* dwarf2/abbrev.h: New file.
	* dwarf2/abbrev.c: New file, from dwarf2read.c.
	* Makefile.in (COMMON_SFILES): Add dwarf2/abbrev.c.

Change-Id: I87911bc5297de4407587ca849fef8e8d19136c30