ada: Revert to old pretty-printing of internal entities for CodePeer

ada: Revert to old pretty-printing of internal entities for CodePeer

Add some defensive code to get pretty-printed CodePeer outputs for ACATS
back to shape. At least some of this code appears to be redundant and
perhaps unnecessary, but we can this up later.

Expression pretty-printer should not be called with
N_Defining_Identifier nodes at all, since they are not expressions.
However, for those that come not from source, CodePeer expects that the
Ident_Image routine will convert the internal names like "xL" to their
corresponding human-readable representation like "x'Accesibility_Level".

gcc/ada/

	* pprint.adb (Expression_Image): Restore some of the old pretty-printing
	for CodePeer.