ada: Fix error and crash on imported function with precondition and 'Base

ada: Fix error and crash on imported function with precondition and 'Base

This fixes a spurious error on an imported function with a precondition
and a parameter declared with a 'Base formal type, and even a crash in
the case where this function is declared in a generic package.

gcc/ada/

	* freeze.adb (Wrap_Imported_Subprogram): Use Copy_Subprogram_Spec
	to copy the spec from the subprogram to the generated subprogram
	body.
	(Freeze_Entity): Do not wrap imported subprograms inside generics.