Fix ICE on nested packed variant record type

Fix ICE on nested packed variant record type

This is a regression present on the mainline and 10 branch: the compiler
aborts on code accessing a component of a packed record type whose type
is a packed discriminated record type with variant part.

gcc/ada/ChangeLog:
	* gcc-interface/utils.c (type_has_variable_size): New function.
	(create_field_decl): In the packed case, also force byte alignment
	when the type of the field has variable size.

gcc/testsuite/ChangeLog:
	* gnat.dg/pack27.adb: New test.
	* gnat.dg/pack27_pkg.ads: New helper.