ada: Fix internal error on imported function with post-condition

The problem, which is also present for an expression function, is that the
function is invoked in the initializing expression of a variable declared
in the same declarative part as the function, which causes the freezing of
its artificial body before the post-condition is analyzed on its spec.

gcc/ada/

	* contracts.adb (Analyze_Entry_Or_Subprogram_Body_Contract): For a
	subprogram body that has no contracts and does not come from source,
	make sure that contracts on its corresponding spec are analyzed, if
	any, before expanding them.