[Ada] Warn on subtype declaration of null range

[Pierre-Marie de Rodat]

This patch adds a warning on a subtype declaration with a
compile-time-known range constraint that is a null range.

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

gcc/ada/

	* sem_res.adb (Resolve_Range): Warn on null range, unless we are
	inside a generic unit or an instance thereof.
	* sem_ch3.adb (Analyze_Subtype_Indication): Minor: avoid double
	negative.
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