a68: fix support for nested access clauses

a68: fix support for nested access clauses

This commit fixes the support for having an access clause as the
controlled clause of another access clause.

Signed-off-by: Jose E. Marchesi <jemarch@gnu.org>
gcc/algol68/ChangeLog

	* a68-parser-top-down.cc (top_down_access): An access clause may
	be nested in another access clause.
	* a68-parser-extract.cc (a68_extract_indicants): Coalesce 'pub'
	symbols.
	(a68_extract_indicants): Nested access are not allowed in module
	texts.
	* a68-parser-bottom-up.cc (expected_module_text): New function.
	(reduce_prelude_packet): Use expected_module_text.
	(a68_bottom_up_error_check): Add comment.

gcc/testsuite/ChangeLog

	* algol68/compile/error-module-nested-access-1.a68: New test.
	* algol68/execute/modules/program-21.a68: Likewise.