[Ada] Attribute Address is not an interfering context in SPARK

[Pierre-Marie de Rodat]

Allow taking the address of a volatile object in SPARK, as it doesn't
cause problems related to interfering contexts.

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

gcc/ada/

	* sem_res.adb (Flag_Object): Ignore prefixes of attribute
	Address.
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