[Ada] array pointers encoded as typedef to fat pointer

[Ada] array pointers encoded as typedef to fat pointer

A recent change in check_typedef caused the following regression,
considering:

        type String_Access is access String;
        S1 : String_Access := null;

Trying to print S1, we get:

        (gdb) print s1
        $1 = (string) (null)

The type name is wrong.  We were expecting:

        (gdb) print s1
        $1 = (string_bug.string_access) (null)

The extensive comment in this patch explains how pointers to arrays
are encoded when the array is a "fat pointer".  What happened is
that the change in check_typedef broke a type-equality test that
we were performing.   The test really was on the fringe, but it
turns out that, for what we're doing, we're really only interested
in the main-type portion of the type.

The patch adjust the check accordingly.

gdb/ChangeLog:

        * ada-lang.c (ada_to_fixed_type): Expand function documentation.
        Return the original type if the main type portions match rather
         than when the type themselves match.