set DECL_SIZE_UNIT for zero-sized fields

set DECL_SIZE_UNIT for zero-sized fields

Zero-sized fields do not get processed by finish_record_type: they're
removed from the field list before and reinserted after, so their
DECL_SIZE_UNIT remains unset, causing the translation of assignment
statements with use_memset_p, in quite unusual circumstances, to use a
NULL_TREE as the memset length.  This patch sets DECL_SIZE_UNIT for
the zero-sized fields, that don't go through language-independent
layout, in language-specific layout.

for  gcc/ada/ChangeLog

	* gcc-interface/decl.c (components_to_record): Set
	DECL_SIZE_UNIT for zero-sized fields.

From-SVN: r276173