Add missing call to obstack_init for ada-lang.c::cache_space

This bit was meant to be merged with the following patch:

    commit 3d9434b5ddc141178be232048e3d944955cc1f91
    Subject: [Ada] Add a symbol lookup cache

... but I forgot :-(. This causes the cache to be undefined, and
with a bit of (bad) luck:

    % gdb
    (gdb) set lang ada
    (gdb) set $xxx := 1
    [SEGV]

gdb/ChangeLog:

        * ada-lang.c (_initialize_ada_language): Initialize
        cache_space obstack.
Wrong type for 'Length result.

Consider the following code:

   type Color is (Black, Red, Green, Blue, White);
   type Primary_Table is array (Color range Red .. Blue) of Boolean;
   Prim : Primary_Table := (True, False, False);

GDB prints the length of arrays in a fairly odd way:

   (gdb) p prim'length
   $2 = blue

The length returned should be an integer, not the array index type,
and this patch fixes this.

gdb/ChangeLog:

	* ada-lang.c (ada_evaluate_subexp): Set the type of the value
	returned by the 'Length attribute to integer.

testsuite/ChangeLog:

	* gdb.ada/tick_length_array_enum_idx: New testcase.
[Ada] Rename some observer callbacks in ada-tasks.c

This patch is mostly cosmetic, avoiding us to use the same callback
names as in ada-lang.c.

gdb/ChangeLog:

        * ada-tasks.c (ada_tasks_new_objfile_observer): Renames
        ada_new_objfile_observer.
        (ada_tasks_normal_stop_observer): Renames ada_normal_stop_observer.
        (_initialize_tasks): Update uses of ada_new_objfile_observer
        and ada_tasks_normal_stop_observer.