Fix regression in Ada do_full_match

An earlier patch to ada-lang.c:do_full_match introduced a subtle
change to the semantics.  The previous code did:

    -  if (strncmp (sym_name, search_name, search_name_len) == 0
    -      && is_name_suffix (sym_name + search_name_len))
    -    return true;
    -
    -  if (startswith (sym_name, "_ada_")

whereas the new code unconditionally skips a leading "_ada_".

The difference occurs if the lookup name itself starts with "_ada_".
In this case, the symbol won't match.

Normally this doesn't seem to be a problem.  However, it caused a
regression on one particular (internal) test case on one particular
platform.

This patch changes the code to handle this case.  I don't know how to
write a reliable test case for this, so no test is included.

2021-01-07  Tom Tromey  <tromey@adacore.com>

	* ada-lang.c (do_full_match): Conditionally skip "_ada_" prefix.