Fix internal error on locally-defined subpools

If the type is derived in the current compilation unit, and Allocate
is not overridden on derivation (as is typically the case with
Root_Storage_Pool_With_Subpools), the entity for Allocate of the
derived type is an alias for System.Storage_Pools.Subpools.Allocate.

The main assertion in gnat_to_gnu_entity fails in this case, since
this is not a definition and Is_Public is false (since the entity
is nested in the same compilation unit).

2020-03-11  Richard Wai  <richard@annexi-strayline.com>

	* gcc-interface/decl.c (gnat_to_gnu_entity): Also test Is_Public on
	the Alias of the entitiy, if is present, in the main assertion.