[Ada] Crash freezing declaration that will raise constraint error

[Ada] Crash freezing declaration that will raise constraint error

When the compiler is built with assertions enabled and processes the
following declarations:

   type Vector_Boolean_Array is array (1 .. 10) of Boolean;
   O2 : constant Vector_Boolean_Array := [for J in 2 => True];

The expression is rewritten by the frontend with an N_Raise_CE node,
which leads to an assertion error at the freezing point of the object
declaration.

gcc/ada/

	* freeze.adb (Freeze_Entity): Protect the call to
	Declaration_Node against entities of expressions replaced by the
	frontend with an N_Raise_CE node.