Fix casting in-memory values of primitive types to const reference

It's currently not possible to cast an in-memory value of a primitive
type to const reference:
```
(gdb) p Q.id
$1 = 42
(gdb) p (int&)Q.id
$2 = (int &) @0x22fd0c: 42
(gdb) p (const int&)Q.id
Attempt to take address of value not located in memory.
```

And if in a function call an argument needs the same kind of casting,
it also doesn't work:
```
(gdb) l f3
39      int f3(const int &i)
40      {
41        return i;
42      }
(gdb) p f3(Q.id)
Attempt to take address of value not located in memory.
```

It's because when the constness of the type changes in a call to
value_cast, a new not_lval value is allocated, which doesn't exist
in the target memory.

Fixed by ignoring const/volatile/restrict qualifications in
value_cast when comparing cast type to original type, so the new
value will point to the same location as the original value:
```
(gdb) p (int&)i
$2 = (int &) @0x39f72c: 1
(gdb) p (const int&)i
$3 = (const int &) @0x39f72c: 1
(gdb) p f3(Q.id)
$4 = 42
```

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=19423


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