Ada: Fix missing call to pretty-printer for fields of records.

Ada: Fix missing call to pretty-printer for fields of records.

Consider the following types:

   type Time_T is record
      Secs : Integer;
   end record;
   Before : Time_T := (Secs => 1384395743);

In this example, we assume that type Time_T is the number of seconds
since Epoch, and so added a Python pretty-printer, to print this
type in a more human-friendly way. For instance:

    (gdb) print before
    $1 = Thu Nov 14 02:22:23 2013 (1384395743)

However, we've noticed that things stop working when this type is
embedded inside another record, and we try to print that record.
For instance, with the following declarations:

   type Composite is record
      Id : Integer;
      T : Time_T;
   end record;
   Afternoon : Composite := (Id => 1, T => (Secs => 1384395865));

    (gdb) print afternoon
    $2 = (id => 1, t => (secs => 1384395865))

We expected instead:

    (gdb) print afternoon
    $2 = (id => 1, t => Thu Nov 14 02:24:25 2013 (1384395865))

This patch fixes the problem by making sure that we try to print
each field via a call to val_print, rather than calling ada_val_print
directly. We need to go through val_print, as the val_print
handles all language-independent features such as calling the
pretty-printer, knowing that ada_val_print will get called eventually
if actual Ada-specific printing is required (which should be the
most common scenario).

And because val_print takes the language as parameter, we enhanced
the print_field_values and print_variant_part to also take a language.
As a bonus, this allows us to remove a couple of references to
current_language.

gdb/ChangeLog:

        * ada-valprint.c (print_field_values): Add "language" parameter.
        Update calls to print_field_values and print_variant_part.
        Pass new parameter "language" in call to val_print instead
        of "current_language".  Replace call to ada_val_print by call
        to val_print.
        (print_variant_part): Add "language" parameter.
        (ada_val_print_struct_union): Update call to print_field_values.

gdb/testsuite/ChangeLog:

        * gdb.ada/pp-rec-component.exp, gdb.ada/pp-rec-component.py,
        gdb.ada/pp-rec-component/foo.adb, gdb.ada/pp-rec-component/pck.adb,
        gdb.ada/pp-rec-component/pck.ads: New files.