Ada: Fix infinite loop on reduction expression with -gnatVa

This is a regression present on the mainline and 16 branch, coming from an
invalid tree sharing in the new implementation of the (complex) analysis of
reduction expressions.

gcc/ada/
	PR ada/126907
	* sem_attr.adb (Resolve_Attribute) <Attribute_Reduce>: Do not call
	Relocate_Node on nodes of the original unanalyzed tree.

gcc/testsuite/
	* gnat.dg/reduce7.adb: New test.