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.