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