Fix regression with partial rep clause on variant record type

Fix regression with partial rep clause on variant record type

It can yield an incorrect layout when there is a partial representation
clause on a discriminated record type with a variant part.

gcc/ada/
	* gcc-interface/decl.c (components_to_record): If the first component
	with rep clause is the _Parent field with variable size, temporarily
	set it aside when computing the internal layout of the REP part again.
	* gcc-interface/utils.c (finish_record_type): Revert to taking the
	maximum when merging sizes for all record types with rep clause.
	(merge_sizes): Put SPECIAL parameter last and adjust recursive calls.