ada: Fix bogus Address Sanitizer stack-buffer-overflow on packed array copy

ada: Fix bogus Address Sanitizer stack-buffer-overflow on packed array copy

The Address Sanitizer considers that the padding at the end of a justified
modular type may be accessed through the object, but it is never accessed
and therefore can always be reused.

gcc/ada/

	* gcc-interface/decl.cc (gnat_to_gnu_entity) <discrete_type>: Set
	the TYPE_JUSTIFIED_MODULAR_P flag earlier.
	* gcc-interface/misc.cc (gnat_unit_size_without_reusable_padding):
	New function.
	(LANG_HOOKS_UNIT_SIZE_WITHOUT_REUSABLE_PADDING): Redefine to above
	function.