[Ada] Fix internal error on predicate aspect with iterator

[Ada] Fix internal error on predicate aspect with iterator

The semantic analysis of predicates involves a fair amount of tree
copying because of both semantic and implementation considerations, and
there is a difficulty with quantified expressions since they declare a
new entity that cannot be shared between the various copies of the tree.

This change implements a specific processing for it in New_Copy_Tree
that subsumes a couple of fixes made earlier for variants of the issue.

gcc/ada/

	* sem_util.ads (Is_Entity_Of_Quantified_Expression): Declare.
	* sem_util.adb (Is_Entity_Of_Quantified_Expression): New
	predicate.
	(New_Copy_Tree): Deal with all entities of quantified
	expressions.
	* sem_ch13.adb (Build_Predicate_Functions): Get rid of
	superfluous tree copying and remove obsolete code.
	* sem_ch6.adb (Fully_Conformant_Expressions): Deal with all
	entities of quantified expressions.