[Ada] Fix bugs in Value_Size clauses and refactor

[Ada] Fix bugs in Value_Size clauses and refactor

gcc/ada/

	* sem_ch13.adb (Analyze_Attribute_Definition_Clause): Combine
	processing of Size and Value_Size clauses. Ensure that
	Value_Size is treated the same as Size, in the cases where both
	are allowed (i.e. the prefix denotes a first subtype).  Misc
	cleanup.
	* einfo-utils.adb (Init_Size): Add assertions.
	(Size_Clause): Return a Value_Size clause if present, instead of
	just looking for a Size clause.
	* einfo.ads (Has_Size_Clause, Size_Clause): Change documentation
	to include Value_Size.
	* sem_ch13.ads, layout.ads, layout.adb: Comment modifications.