This patch adds several legality checks on calls to an instance of the
predefined Generic_Dispatchin_Constructor. The following three tests are
performed:

a) The tag argument is defined, i.e. is not No_Tag.

b) The  tag is not that of an abstract type.

c) The accessibility level of the type denoted by the tag is no greater than
that of the specified constructor function.

Tested in  ACATS 4.0H C390012.

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

2015-11-12  Ed Schonberg  <schonberg@adacore.com>

	* exp_intr.adb: Add legality checks on calls to a
	Generic_Dispatching_Constructor: the given tag must be defined,
	it cannot be the tag of an abstract type, and its accessibility
	level must not be greater than that of the constructor.