[gdb/exp] Fix cast handling for indirection

Consider a test-case compiled without debug info, containing:
...
char a = 'a';

char *
a_loc (void)
{
  return &a;
}
...

We get:
...
(gdb) p (char)*a_loc ()
Cannot access memory at address 0x10
...

There's a bug in unop_ind_base_operation::evaluate that evaluates
"(char)*a_loc ()" the same as:
...
(gdb) p (char)*(char)a_loc ()
Cannot access memory at address 0x10
...

Fix this by instead evaluating it the same as:
...
(gdb) p (char)*(char *)a_loc ()
$1 = 97 'a'
...

Tested on x86_64-linux.

PR exp/31693
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31693