[Ada] Failure compiling "for ... of" loop over a slice

[Ada] Failure compiling "for ... of" loop over a slice

In some cases involving a "for ... of" loop (not to be confused with the
more common "for ... in" loop) iterating over a slice, compilation would
fail with an internal compiler error.

gcc/ada/

	* sem_util.adb (Get_Actual_Subtype): If a new subtype is built,
	do not freeze it if Expander_Active is False. The idea here is
	to avoid generating an unwanted Freeze_Node for a subtype that
	has been conjured up solely for purposes of preanalysis.