ada: Fix internal error on array constant in expression function

ada: Fix internal error on array constant in expression function

This happens when the peculiar check emitted by Check_Large_Modular_Array
is applied to an object whose actual subtype is an itype with dynamic size,
because the first reference to the itype in the expanded code may turn out
to be within the raise statement, which is problematic for the eloboration
of this itype by the code generator at library level.

gcc/ada/

	* freeze.adb (Check_Large_Modular_Array): Fix head comment, use
	Standard_Long_Long_Integer_Size directly and generate a reference
	just before the raise statement if the Etype of the object is an
	itype declared in an open scope.