[Ada] Fix regression with partial rep clause on variant record type

[Eric Botcazou]

It is present on the mainline, 10 and 9 branches, and can yield an incorrect
layout when there is a partial representation clause on a discriminated record
type with a variant part.

Tested on x86-64/Linux, applied on mainline, 10 and 9 branches.


2021-02-03  Eric Botcazou  <ebotcazou@adacore.com>

	* 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.

-- 
Eric Botcazou
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