[Ada] Fix internal error on package instantiation on private type

[Ada] Fix internal error on package instantiation on private type

This fixes an assertion failure in gigi triggered by the instantiation
of a generic package, in a visible part of another package, done on a
private type whose full view is a type derived from a scalar or an
access type.

The problem is that the front-end creates and inserts two different
freeze nodes in the expanded tree for the partial and the full views of
the private subtype created by the instantiation, which is not correct:
partial and full views of a given (sub)type must point to the same
freeze node, if any.

The patch also adds an assertion checking this property in the front-end
so as to catch the inconsistency higher in the chain.

2018-12-03  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* freeze.adb (Freeze_Entity): Do not freeze the partial view of
	a private subtype if its base type is also private with delayed
	freeze before the full type declaration of the base type has
	been seen.
	* sem_ch7.adb (Preserve_Full_Attributes): Add assertion on
	freeze node.

gcc/testsuite/

	* gnat.dg/generic_inst2.adb, gnat.dg/generic_inst2.ads,
	gnat.dg/generic_inst2_c.ads: New testcase.

From-SVN: r266754