[Ada] Fix handling of Ada 83 OUT parameter rule

[Pierre-Marie de Rodat]

The current code was climbing the tree manually assuming that an array
attribute (e.g. 'First) would appear immediately, which isn't the case
for e.g. a selected component expression.

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

2020-06-12  Arnaud Charlet  <charlet@adacore.com>

gcc/ada/

	* sem_res.adb (Resolve_Entity_Name): Fix handling of expressions
	containing array attributes wrt Ada 83 detection.
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