[Ada] Reject illegal syntax in pragma Contract_Cases

[Ada] Reject illegal syntax in pragma Contract_Cases

gcc/ada/

	* contracts.adb (Analyze_Entry_Or_Subprogram_Contract,
	Analyze_Subprogram_Body_Stub_Contract): Fix missing references
	to Subprogram_Variant where similar references to Contract_Cases
	are present.
	* sem_prag.adb (Analyze_Contract_Case, Analyze_Variant): Check
	that aggregate parameter has no expressions.
	(Analyze_Pragma): Replace Contract_Cases with Subprogram_Variant
	in a copy-pasted comment.