(Ada) fix handling of expression with parameterless function call

(Ada) fix handling of expression with parameterless function call

Consider the following function, which takes no parameter and returns
an integer:

    function Something return Integer;

For the purpose of this discussion, our function has been implemented
to always return 124:

    function Something return Integer is
    begin
       return 124;
    end Something;

In Ada, such function can been called without using the parentheses.
For instance, in the statement below, variable My_Value is assigned
the returned value from the call to Something:

    My_Value := Something;

The Ada expression interpeter in GDB supports this case, as we can
see below:

    (gdb) print something
    $1 = 124

However, we get fairly strange results when trying to use this feature
as part of a larger expression. For instance:

    (gdb) print something + 1
    $2 = 248

The problem occurs while doing the resolution pass of the expression.
After prefixying the expression, we obtain the following expression:

    0  BINOP_ADD
    1    OP_VAR_VALUE          Block @0x2021550, symbol @0x20213a0 (pck.something)
    5    OP_LONG               Type @0x1e3c170 (int), value 1 (0x1)

The resolution pass is then expected to remove the OP_VAR_VALUE
entry, and replace it with an OP_FUNCALL. This is what the call
to replace_operator_with_call in ada-lang.c::resolve_subexp is
expected to do:

      if (deprocedure_p
          && (TYPE_CODE (SYMBOL_TYPE (exp->elts[pc + 2].symbol))
              == TYPE_CODE_FUNC))
        {
          replace_operator_with_call (expp, pc, 0, 0,
                                      exp->elts[pc + 2].symbol,
                                      exp->elts[pc + 1].block);
          exp = expp->get ();
        }

The problem is that we're passing OPLEN (zero -- 4th parameter in
the call), and so replace_operator_with_call ends up removing zero
element from our expression, and inserting the corresponding OP_FUNCALL
instead. As a result, instead of having the OP_LONG (1) as the second
argument of the BINOP_ADD, it is now the OP_VAR_VALUE that we were
meant to replace. That OP_VAR_VALUE then itself gets transformed into
an OP_FUNCALL, with the same issue, and eventually, the resolved
expression now looks like this:

     0  BINOP_ADD
     1    OP_FUNCALL            Number of args: 0
     4      OP_VAR_VALUE          Block @0x2021550, symbol @0x20213a0 (pck.something)
     8    OP_FUNCALL            Number of args: 0
    11      OP_VAR_VALUE          Block @0x2021550, symbol @0x20213a0 (pck.something)
    15  OP_VAR_VALUE          Block @0x2021550, symbol @0x20213a0 (pck.something)
    19  OP_LONG               Type @0x1e3c170 (int), value 1 (0x1)

This explains why we get twice the result of the function call
instead of its value plus one. The extra entries in the expression
at the end are just ignored.

This patch fixes the issue by calling replace_operator_with_call
with the correct OPLEN equal to the size of an OP_VAR_VALUE (4).

gdb/ChangeLog:

        * ada-lang.c (resolve_subexp): Pass correct OPLEN in call to
        replace_operator_with_call.

gdb/testsuite/ChangeLog:

        * gdb.ada/expr_with_funcall: New testcase.