[Ada] Crash on declaration with aggregate for unchecked union

[Pierre-Marie de Rodat]

This patch fixes a compiler abort on an object declaration whose
expression is an aggregate for an unchecked union type. A declaration
for an object of such a type is otherwise treated as a renaming of the
expression (typically an object of the type), but this does work for an
aggregate, which must be resolved and handled as an initial value of
a regular declaration.

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

gcc/ada/

	* sem_ch3.adb (Analyze_Object_Declaration): If the type is an
	Unchecked_Union, and the expression is an aggregate. complete
	the analysis and resolution of the aggregate, and treat like a
	regular object declaration, instead of as a renaming
	declarattion.
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