(Ada) fix "error in expression" when using watch -location command

The "watch -l EXPR" command with the language set to Ada currently
fails with the following error:

     (gdb) watch -l global_var
     Error in expression, near ` 0x000000000062d2d8'.

The error occurs because GDB internally translate the request into
a watchpoint on a different expression: "* (TYPE *) ADDR" where
TYPE and ADDR are the type and the address of the object returned
by the expression's evaluation (resp.). So, in the example above,
global_var being an integer stored at 0x000000000062d2d8, GDB tries
to set a watchpoint on "* (integer *) 0x000000000062d2d8", which
fails, because we try to parse this expression with Ada, when
in fact it is not valid.

This patch fixes the issue by implementing the la_watch_location_expression
language method, using a syntax that the Ada parser recognizes
("{TYPE} ADDR").

gdb/ChangeLog:

	* ada-lang.c (ada_watch_location_expression): New function.
        (ada_language_defn): Set la_watch_location_expression to
        ada_watch_location_expression.

gdb/testsuite/ChangeLog:

        * gdb.ada/watch_minus_l: New testcase.

1 files changed, 22 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.ada/watch_minus_l/pck.ads b/gdb/testsuite/gdb.ada/watch_minus_l/pck.ads
new file mode 100644
index 0000000..bf19b16
--- /dev/null
+++ b/gdb/testsuite/gdb.ada/watch_minus_l/pck.ads
@@ -0,0 +1,22 @@
+--  Copyright 2018 Free Software Foundation, Inc.
+--
+--  This program is free software; you can redistribute it and/or modify
+--  it under the terms of the GNU General Public License as published by
+--  the Free Software Foundation; either version 3 of the License, or
+--  (at your option) any later version.
+--
+--  This program is distributed in the hope that it will be useful,
+--  but WITHOUT ANY WARRANTY; without even the implied warranty of
+--  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+--  GNU General Public License for more details.
+--
+--  You should have received a copy of the GNU General Public License
+--  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+with System;
+
+package Pck is
+   Global_Var : Integer := 0;
+
+   procedure Do_Nothing (A : System.Address);
+end Pck;