[Ada] Diagnose illegal overriding indicators in protected bodies

[Pierre-Marie de Rodat]

This patch adds checks to reject illegal overriding indicators that
appear on subprogram declarations in protected bodies, where such
a subprogram cannot override any operation. Fixes ACATS test B831006.

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

2020-06-12  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_ch6.adb (Check_Overriding_Indicatior): Reject an
	overriding indicator on a subprogram declared within a protected
	body.
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