a68: avoid coalescing of stmt_lists in a68_lower_unit_list

At it happens, a68_lower_unit_list collects units as members of a
stmt_list, lowering them and appending them.  Problem is, stmt_lists
get coalesced when appended (or prepended) to another stmt_list.

One of the units that may lower in a stmt_list are generators, and the
coalescing manifests itself when the generators are found in
collateral clauses.

This patch puts in place a temporary workaround for this, which is to
wrap the stmt_list into a NOP_EXPR.  This avoid the coalescing, but a
less hackish solution will probably consist on changing the way
unit_lists get collected instead.

Signed-off-by: Jose E. Marchesi <jemarch@gnu.org>

gcc/algol68/ChangeLog

	* a68-low-generator.cc (a68_low_generator): Wrap the resulting
	stmt_list into a nop_expr.

gcc/testsuite/ChangeLog

	* algol68/execute/gen-in-constructor-1.a68: New test.
	* algol68/execute/gen-in-constructor-2.a68: Likewise.