GDB/MI: crash printing "_task" (Ada) argument

In GDB/MI mode, trying to print the arguments of the frame corresponding
to the body of a task ("-stack-list-arguments 1") causes the debugger to
crash.

This is because the compiler adds an implicit argument to that task body
called "_task". mi/mi-cmd-stack.c:list_args_or_locals, which is
responsible for printing the value of our arguments, finds that our
"_task" symbol is an argument, and thus tries to fing the non-argument
equivalent:

              if (SYMBOL_IS_ARGUMENT (sym))
                sym2 = lookup_symbol (SYMBOL_NATURAL_NAME (sym),
                                      block, VAR_DOMAIN,
                                      (int *) NULL);

Unfortunately, it tries using the natural name, which doesn't always
work for Ada parameters, in particular those who are internally-
generated. In our case, The "_task" parameter's natural name is
"<_task>", and that symbol does not exist.  So sym2 is NULL, thus
causing the crash a little later on when trying to dereference it.
We should be using the symbol linkage name in this case, the same
way iterate_over_block_arg_vars already does.

gdb/ChangeLog:

        * mi/mi-cmd-stack.c (list_args_or_locals): For argument symbols,
        use SYMBOL_LINKAGE_NAME to find the corresponding non-argument
        symbol.  Add assertion that sym2 is never NULL.

gdb/testsuite/ChangeLog:

        * gdb.ada/mi_task_arg: New testcase.

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