Treat Character as a discrete type in Ada

Treat Character as a discrete type in Ada

A user noticed that gdb would assert when printing a certain array
with array-indexes enabled.  This turned out to be caused by the array
having an index type of Character, which is completely valid in Ada.
This patch changes the Ada support to recognize Character as a
discrete type, and adds some tests.

Because this is Ada-specific and was also reviewed internally, I am
checking it in.