[Ada] Fix spurious instantiation error on private record type

[Pierre-Marie de Rodat]

This change was initially aimed at fixing a spurious instantiation error
due to a disambiguation issue which happens when a generic unit with two
formal type parameters is instantiated on a single actual type that is
private.

The compiler internally sets the Is_Generic_Actual_Type flag on the
actual subtypes built for the instantiation in order to ease the
disambiguation, but it would fail to set it on the full view if the
subtypes are private.  The change makes it so that the flag is properly
set and reset on the full view in this case.

But this uncovered an issue in Subtypes_Statically_Match, which was
relying on a stalled Is_Generic_Actual_Type flag set on a full view
outside of the instantiation to return a positive answer.  This bypass
was meant to solve an issue arising with a private discriminated record
type whose completion is a discriminated record type itself derived from
a private discriminated record type, which is used as actual type in an
instantiation in another unit, and the instantiation is used in a child
unit of the original unit.  In this case, the private and full views of
the generic actual type are swapped in the child unit, but there was a
mismatch between the chain of full and underlying full views of the
private discriminated record type and that of the generic actual type.

This secondary issue is solved by avoiding to skip the full view in the
preparation of the completion of the private subtype and by directly
constraining the underlying full view of the full view of the base type
instead of building an underlying full view from scratch.

Tested on x86_64-pc-linux-gnu, committed on trunk

2019-08-13  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* sem_ch3.adb (Build_Underlying_Full_View): Delete.
	(Complete_Private_Subtype): Do not set the full view on the
	private subtype here.  If the full base is itself derived from
	private, do not re-derive the parent type but instead constrain
	an existing underlying full view.
	(Prepare_Private_Subtype_Completion): Do not get to the
	underlying full view, if any.  Set the full view on the private
	subtype here.
	(Process_Full_View): Likewise.
	* sem_ch12.adb (Check_Generic_Actuals): Also set
	Is_Generic_Actual_Type on the full view if the type of the
	actual is private.
	(Restore_Private_Views): Also reset Is_Generic_Actual_Type on
	the full view if the type of the actual is private.
	* sem_eval.adb (Subtypes_Statically_Match): Remove bypass for
	generic actual types.

gcc/testsuite/

	* gnat.dg/generic_inst10.adb, gnat.dg/generic_inst10_pkg.ads:
	New testcase.
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