[Ada] Move overriding rename error message from declaration to use

[Pierre-Marie de Rodat]

Posting the error message on the declaration of the renamed subprogram
is more confusing than posting the message on the name of the renamed
subprogram in the renaming.

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

gcc/ada/

	* sem_ch3.adb (Check_Abstract_Overriding): Post error message on
	renaming node.
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