Allow calling of user-defined function call operators

Currently it's not possible to call user-defined function call
operators, at least not without specifying operator() directly:
```
(gdb) l 1
1       struct S {
2         int operator() (int x) { return x + 5; }
3       };
4
5       int main () {
6         S s;
7
8         return s(23);
9       }
(gdb) p s(10)
Invalid data type for function to be called.
(gdb) p s.operator()(10)
$1 = 15
```

This now looks if an user-defined call operator is available when
trying to 'call' a struct value, and calls it instead, making this
possible:
```
(gdb) p s(10)
$1 = 15
```

The change in operation::evaluate_funcall is to make sure the type
fields are only used for function types, only they use them as the
argument types.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=12213


Approved-By: Tom Tromey <tom@tromey.com>