This patch fixes a crash found on "p *(type *)0x1234" when using "set
debug expression 1".

While casting works as expected with expression debugging turned off,
this seems to be an indication that I'm infact doing something wrong
in the building of the expression.

(gdb) print *(int*)(0x0)
Dump of expression @ 0x12db320, before conversion to prefix form:
    Language d, 11 elements, 16 bytes each.
    Index                Opcode         Hex Value  String Value
        0               OP_TYPE  87  W...............
        1    <unknown 20114800>  20114800  p.2.............
        2               OP_TYPE  87  W...............
        3               OP_LONG  38  &...............
        4    <unknown 19696640>  19696640  ..,.............
        5               OP_NULL  0  ................
        6               OP_LONG  38  &...............
        7             UNOP_CAST  51  3...............
        8    <unknown 20114800>  20114800  p.2.............
        9             UNOP_CAST  51  3...............
       10              UNOP_IND  61  =...............
Dump of expression @ 0x12db320, after conversion to prefix form:
Expression: `*(int *) 0'
    Language d, 11 elements, 16 bytes each.


        0  UNOP_IND
        1    UNOP_CAST             Type @0x132ed70 (int *)
        4      OP_LONG               Type @0x12c8c00 (int), value 0 (0x0)
        8  <unknown 20114800>    Unknown format
        9  UNOP_CAST             Type @0x3d (Segmentation fault


Looks like using UNOP_CAST_TYPE is the right thing to do here, as the
TypeExp handler has wrapped the type around a pair of OP_TYPE opcodes.