[Ada] Perform object rewriting as renaming only in the expander

[Ada] Perform object rewriting as renaming only in the expander

The rewriting as renaming optimization for object declarations is done
partly during analysis, guarded with Expander_Active, and partly during
expansion, so it makes sense to do it entirely during expansion.

This merges the two cases and removes obsolete or unnecessary conditions
guarding the transformation in the process.

gcc/ada/

	* exp_ch3.adb (Expand_N_Object_Declaration): Rewrite as a renaming
	for any nonaliased local object with nominal unconstrained subtype
	originally initialized with the result of a function call that has
	been rewritten as the dereference of a reference to the result.
	* sem_ch3.adb (Analyze_Object_Declaration): Do not do it here