[Ada] Fix insertion of declaration inside quantified expression

[Pierre-Marie de Rodat]

When the evaluation of the subtype_indication for the
iterator_specification of a quantified_expression leads to the insertion
of a type declaration, this should be done with Insert_Action instead of
Insert_Before.

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

gcc/ada/

	* sem_ch5.adb (Analyze_Iterator_Specification): Use
	Insert_Action when possibly inside an expression.
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