Inline abbrev lookup

Inline abbrev lookup

Profiling showed that calls to abbrev_table::lookup_abbrev were "too
visible".  As these are just forwarding calls to the hash table, this
patch inlines the lookup.  Also, htab_find_with_hash is used, avoiding
another call.

The run previous to this had times of (see the first patch in the
series for an explanation):

gdb    1.69
libxul 2.02
Ada    2.52

This patch improves the times to:

gdb    1.64
libxul 1.99
Ada    2.47

gdb/ChangeLog
2020-05-27  Tom Tromey  <tromey@adacore.com>

	* dwarf2/abbrev.h (struct abbrev_table) <lookup_abbrev>: Inline.
	Use htab_find_with_hash.
	<add_abbrev>: Remove "abbrev_number" parameter.
	* dwarf2/abbrev.c (abbrev_table::add_abbrev): Remove
	"abbrev_number" parameter.  Use htab_find_slot_with_hash.
	(hash_abbrev): Add comment.
	(abbrev_table::lookup_abbrev): Move to header file.
	(abbrev_table::read): Update.