[Ada] Refine type of a counter-like variable

[Pierre-Marie de Rodat]

A local variable that is used as a counter (which is clear from both its
comment and its used) will only be assigned with natural numbers. This
is now reflected in its type. Code cleanup only; semantics is
unaffected.

Tested on x86_64-pc-linux-gnu, committed on trunk

2020-06-11  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* sem_aggr.adb (Resolve_Record_Aggregate): Refine type of
	Others_Box.
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