[Ada] Incorrect code for anonymous access-to-function with convention C

This patch fixes a bug where the compiler generates incorrect code for a
call via an object with convention C, whose type is an anonymous
access-to-function type.

gcc/ada/

	* einfo-utils.adb (Set_Convention): Call Set_Convention
	recursively, so that Set_Can_Use_Internal_Rep is called (if
	appropriate) on the anonymous access type of the object, and its
	designated subprogram type.
	* sem_ch3.adb (Access_Definition): Remove redundant call to
	Set_Can_Use_Internal_Rep.