[Ada] Misleading warning on variable not assigned

[Pierre-Marie de Rodat]

This patch removes a warning on a referenced entity with no explicit
prior assignment, if the type of the entity has
Preelaborable_Initialixation, such as Exception_Occurrence.

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

2019-07-22  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_warn.adb (Check_References): Do not emit s warning on a
	referenced entity with no explicit assignment if the type of the
	entity has Preelaborable_Initialixation, such as
	Exception_Occurrence.

gcc/testsuite/

	* gnat.dg/warn25.adb: New testcase.
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