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>

1 files changed, 2 insertions, 1 deletions

diff --git a/gdb/valops.c b/gdb/valops.c
index 1a943f0..52cce28 100644
--- a/gdb/valops.c
+++ b/gdb/valops.c
@@ -411,7 +411,8 @@ value_cast (struct type *type, struct value *arg2)
      In this case we want to preserve the LVAL of ARG2 as this allows the
      resulting value to be used in more places.  We do this by calling
      VALUE_COPY if appropriate.  */
-  if (types_deeply_equal (arg2->type (), type))
+  if (types_deeply_equal (make_unqualified_type (arg2->type ()),
+			  make_unqualified_type (type)))
     {
       /* If the types are exactly equal then we can avoid creating a new
 	 value completely.  */