Move the using directives to buildsym_compunit

Move the using directives to buildsym_compunit

This moves the globals local_using_directives and
global_using_directives to be members of buildsym_compunit, and adds
the necessary accessors.

gdb/ChangeLog
2018-07-16  Tom Tromey  <tom@tromey.com>

	* dwarf2read.c (using_directives, read_func_scope)
	(read_lexical_block_scope): Update.
	* cp-namespace.c (cp_scan_for_anonymous_namespaces): Update.
	* buildsym.h (local_using_directives, global_using_directives):
	Don't declare.
	(get_local_using_directives, set_local_using_directives)
	(get_global_using_directives): Declare.
	* buildsym.c (struct buildsym_compunit) <m_local_using_directives,
	m_global_using_directives>: New members.
	(finish_block_internal, prepare_for_building)
	(reset_symtab_globals, end_symtab_get_static_block)
	(push_context): Update.
	(get_local_using_directives, set_local_using_directives)
	(get_global_using_directives): New functions.
	(buildsym_init): Update.