Consider the following type:

   type Char_Enum_Type is ('A', 'B', 'C', 'D');

If the compiler generates a Char_Enum_Type typedef in the debugging
information, the debugger fails in the following case:

   (gdb) p Char_Enum_Type'('B')
   $1 = 66

For our type, the underlying value of 'B' is actually 1, not 66
(ASCII 'B').  We are failing this case because we were not handling
typedef to enum types before.  This patch fixes this.

gdb/ChangeLog:

        * ada-exp.y (convert_char_literal): Handle typedef types.

gdb/testsuite/ChangeLog:

        * gdb.ada/char_enum: New testcase.