dwarf2read.c:mapped_index, use gdb::array_view, simplify symbol table

dwarf2read.c:mapped_index, use gdb::array_view, simplify symbol table

This replaces a couple ptr+size pairs with gdb::array_view in the
.gdb_index code, and simplifies things by using an aggregate for the
type of the symbol table hash bucket instead of having to consider the
distinction between size of table vs number of slots and access name
vs vec by index.

gdb/ChangeLog:
2017-12-08  Pedro Alves  <palves@redhat.com>

	* dwarf2read.c (mapped_index::symbol_table_slot): New.
	(mapped_index::address_table): Now a gdb::array_view of const
	gdb_byte.
	(mapped_index::symbol_table): Now a gdb::array_view of
	symbol_table_slot.
	(mapped_index::address_table_size)
	(mapped_index::symbol_table_slots): Delete.
	(create_addrmap_from_index): Adjust.
	(find_slot_in_mapped_hash): Adjust.
	(read_index_from_section): Adjust.
	(dwarf2_read_index): Adjust.