[Ada] gdb.ada/complete.exp failure on x86_64-windows

[Ada] gdb.ada/complete.exp failure on x86_64-windows

Using the example in gdb.ada/complete.exp, the following command
on x86_64-windows returns one unwanted completion choice :

    (gdb) complete p pck
    p <pck_E>>
    [all following completions entries snipped, all expected]

I tracked down this suprising entry to a minimal symbol whose name
is ".refptr.pck_E". The problem occurs while trying to see if
this symbol matches "pck" when doing wild-matching as we are doing
here:

  /* Second: Try wild matching...  */

  if (!match && wild_match_p)
    {
      /* Since we are doing wild matching, this means that TEXT
         may represent an unqualified symbol name.  We therefore must
         also compare TEXT against the unqualified name of the symbol.  */
      sym_name = ada_unqualified_name (ada_decode (sym_name));
      if (strncmp (sym_name, text, text_len) == 0)
        match = 1;
    }

What happens is that ada_decode correctly identifies the fact that
SYM_NAME (".refptr.pck_E") is not following any GNAT encoding, and
therefore returns that same name, but bracketed: "<.refptr.pck_E>".
This is the convention we use for telling GDB that the decoded name
is not a real Ada name - and therefore should not be encoded for
operations such as name matching, symbol lookups, etc. So far, so good.

Next is the call to ada_unqualified_name, which unfortunately does
not notice that the decoded name it is being given isn't a natural
symbol, and just blindly strips everything up to the last do, returning
"pck_E>". And of course, "pck_E>" matches "pck" now, and so we end
up accepting this symbol as a match.

This patch fixes the problem by making ada_unqualified_name a little
smarter by making sure that the given decoded symbol name does not
start with '<'.

gdb/ChangeLog:

        * ada-lang.c (ada_unqualified_name): Return DECODED_NAME if
        it starts with '<'.

Tested on x86_64-windows using AdaCore's testsuite as well as
on x86_64-linux.