ada: Fix crash on legality check for initialization of implicit constructor

ada: Fix crash on legality check for initialization of implicit constructor

This patch fixes a crash occurring during the legality check of the Initialize
aspect when the constructor is implicitly created by the compiler, e.g., the
default copy constructor. In such case, Corresponding_Spec is not available, the
Specification field must be used instead.

gcc/ada/ChangeLog:

	* sem_ch13.adb (Check_Constructor_Initialization_Expression): The first
	parameter of an implicit constructor comes from Specification, not
	Corresponding_Spec.