(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.