ada: Cleanup preanalysis of static expressions (part 4)

Fix regression in the SPARK 2014 testsuite.

gcc/ada/ChangeLog:

	* sem_util.adb (Build_Actual_Subtype_Of_Component): No action
	under preanalysis.
	* sem_ch5.adb (Set_Assignment_Type): If the right-hand side contains
	target names, expansion has been disabled to prevent expansion that
	might move target names out of the context of the assignment statement.
	Restore temporarily the current compilation mode so that the actual
	subtype can be built.