[Ada] Emit error messages for null/generic nonreturning procedures

[Pierre-Marie de Rodat]

This commit adds checks for rule 6.5.1 4/3 of the Ada RM which declares
nonreturning procedures and nonreturning generic procedure instances
illegal.

The reason this check is performed in sem_prag.adb rather than in
Check_Returns in sem_ch6.adb is that generic procedure instances won't
have their No_Return flag set yet when Check_Returns is called.

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

gcc/ada/

	* sem_prag.adb (Analyze_Pragma): declare new Check_No_Return
	function and call it.
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