Add proper handling for internal functions and STT_GNU_IFUNC symbols in Ada mode.

Add proper handling for internal functions and STT_GNU_IFUNC symbols in Ada mode.

The code for handling calls to internal functions (esp., Python
functions) and for handling STT_GNU_IFUNC had not been added to the Ada
expression evaluator.  This change adapts them from eval.c.

gdb/Changelog:

        * ada-lang.c (ada_evaluate_subexp): Add cases for
          TYPE_CODE_INTERNAL_FUNCTION and for TYPE_GNU_IFUNC, following
          their treatment in eval.c.