dwarf2read.c: Rewrite/simplify mock_mapped_index

dwarf2read.c: Rewrite/simplify mock_mapped_index

Now that dw2_expand_symtabs_matching_symbol works with the abstract
mapped_index_base, we can make mock_mapped_index inherit
mapped_index_base too instead of having it pretend to be a real
.gdb_index table.

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

	* dwarf2read.c (mock_mapped_index): Reimplement as an extension of
	mapped_index_base.
	(check_match): Adjust to use mock_index directly.
	(check_find_bounds_finds)
	(test_mapped_index_find_name_component_bounds): Adjust to work
	with a mapped_index_base.