[Ada] Build full derivation for private concurrent type

[Ada] Build full derivation for private concurrent type

This extends the processing done for the derivation of private
discriminated types to concurrent types, which is now required because
this derivation is no longer redone when a subtype of the derived
concurrent type is built.

This increases the number of entities generated internally in the
compiler but this case is sufficiently rare as not to be a real concern.

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

gcc/ada/

	* sem_ch3.adb (Build_Derived_Concurrent_Type): Add a couple of
	local variables and use them.  When the derived type fully
	constrains the parent type, rewrite it as a subtype of an
	implicit (unconstrained) derived type instead of the other way
	around.
	(Copy_And_Build): Deal with concurrent types and use predicates.
	(Build_Derived_Private_Type): Build the full derivation if
	needed for concurrent types too.
	(Build_Derived_Record_Type): Add marker comment.
	(Complete_Private_Subtype): Use predicates.

gcc/testsuite/

	* gnat.dg/discr56.adb, gnat.dg/discr56.ads,
	gnat.dg/discr56_pkg1.adb, gnat.dg/discr56_pkg1.ads,
	gnat.dg/discr56_pkg2.ads: New testcase.

From-SVN: r274359