[Ada] Simplify and reuse Is_Concurrent_Interface

[Pierre-Marie de Rodat]

Code cleanup; semantics is unaffected.

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

gcc/ada/

	* exp_ch3.adb (Stream_Operation_OK): Reuse
	Is_Concurrent_Interface.
	* sem_ch3.adb (Analyze_Interface_Declaration,
	Build_Derived_Record_Type): Likewise.
	* sem_ch6.adb (Check_Limited_Return): Likewise.
	* sem_util.adb (Is_Concurrent_Interface): Don't call
	Is_Interface because each of the Is_Protected_Interface,
	Is_Synchronized_Interface and Is_Task_Interface calls it anyway.
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