DWARF 5 support: Handle line table and file indexes

DWARF 5 support: Handle line table and file indexes

*  Fix handling of file and directory indexes in line tables; in DWARF 5 the
indexes are zero-based. Make file_names field private to abstract this detail
from the clients. Introduce file_names, is_valid_file_index and
file_names_size methods. Reflect these changes in clients.
*  Handle DW_FORM_data16 in read_formatted_entries; it is used to record MD5
of the file entries in DWARF 5.
*  Fix a bug in line header parsing that calculates the length of the header
incorrectly. (Seemingly this manifests itself only in DWARF 5).

Tested with CC=/usr/bin/gcc (version 8.3.0) against master branch (also with
-gsplit-dwarf and -gdwarf-4 flags) and there was no increase in the set of
tests that fails. (gdb still cannot debug a 'hello world' program with DWARF 5,
so for the time being, this is all we care about).

This is part of an effort to support DWARF 5 in gdb.

gdb/ChangeLog:

	* dwarf2read.c (dir_index): Change type.
	(file_name_index): Likewise.
	(line_header::include_dir_at): Change comment and implementation on
	whether it is DWARF 5.
	(line_header::is_valid_file_index): New function.
	(line_header::file_name_at): Change comment and implementation on
	whether it is DWARF 5.
	(line_header::file_names): Change to private field renamed as
	m_file_names and introduce a new accessor method.
	(line_header::file_names_size): New method.
	(line_header::include_dirs): Change to private field and rename as
	m_include_dirs.
	(dw2_get_file_names_reader): Define local var at a smaller scope and
	reflect API change.
	(dwarf2_cu::setup_type_unit_groups): Reflect API change.
	(process_structure_scope): Likewise.
	(line_header::add_include_dir): Change message and reflect renaming.
	(line_header::add_file_name): Likewise.
	(read_formatted_entries): Handle DW_FORM_data16.
	(dwarf_decode_line_header): Fix line header length calculation.
	(psymtab_include_file_name): Change comment and API.
	(lnp_state_machine::m_file): Update comment and reflect type change.
	(lnp_state_machine::record_line): Reflect type change.
	(dwarf_decode_lines): Reflect API change.
	(file_file_name): Likewise.
	(file_full_name): Likewise.