[Ada] Suppress Base_Address call on init of activation record components

[Ada] Suppress Base_Address call on init of activation record components

2019-12-13  Gary Dismukes  <dismukes@adacore.com>

gcc/ada/

	* exp_attr.adb (Expand_N_Attribute_Reference,
	Attribute_Address): Suppress generation of a call to
	Base_Address in the case where the Address attribute is being
	used to initialize a corresponding component of an activation
	record.
	(Is_Unnested_Component_Init): New function to determine whether
	an attribute reference for Address is used to initialized a
	component of an activation record object that corresponds to the
	object denoted by the prefix of the attribute (an assignment
	used in support of unnesting for back ends like LLVM).

From-SVN: r279353