[Ada] Fix spurious error on checking of null Abstract_State

[Pierre-Marie de Rodat]

A declaration of an object inside a declare block of elaboration code
should not count as hidden state of the package. Nor should declarations
inside a task or an entry. Now fixed.

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

2020-05-25  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* sem_util.adb (Check_No_Hidden_State): Stop propagation at
	first block/task/entry.
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