[Ada] Simplify iteration over components

[Pierre-Marie de Rodat]

Instead of iterating with First_Entity/Next_Entity and then detecting
components with Ekind, it feels simpler to iterate with First_Component/
Next_Component.

Tested on x86_64-pc-linux-gnu, committed on trunk

2020-06-05  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* einfo.adb: Minor reformatting.
	* sem_util.adb (Has_Full_Default_Initialization,
	Is_Partially_Initialized_Type, Caller_Known_Size_Record,
	Large_Max_Size_Mutable): Iterate with
	First_Component/Next_Component; rename Ent to Comp.
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