c++: modules and debug marker stmts

21_strings/basic_string/operations/contains/nonnull.cc was failing because
the module was built with debug markers and the testcase was built not
expecting debug markers, so we crashed in lower_stmt.  Let's accommodate
this by discarding debug marker statements we don't want.

gcc/cp/ChangeLog:

	* module.cc (trees_in::core_vals) [STATEMENT_LIST]: Skip
	DEBUG_BEGIN_STMT if !MAY_HAVE_DEBUG_MARKER_STMTS.