Handle enumerators in name associations in Ada

Handle enumerators in name associations in Ada

assoc.exp does this:

    print pck.value := (Left => 3, Center => 7, Pck.Right => 2)

However, the test case is constructed so that "Center" has multiple
meanings: it might name the enumerator here:

    type Posn is (Left, Center, Right);

or it might refer to this variable:

   Center : Pck.Posn := Pck.Right;

The correct answer in this case is to pick the enumerator, because
that is the array's index type.

Originally I didn't fix this problem and instead used a kfail.
However, this same problem turned up again (in a slightly different
form) when the libgnat debuginfo was installed.

This patch fixes the bug by introducing new code in
ada_name_association::assign to handle the enumeration case.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33896
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33898
Tested-By: Tom de Vries <tdevries@suse.de>