[Clang][BPF] Type print btf_type_tag properly

When running bcc tool execsnoop ([1]) which is built with latest llvm,
I hit the following error:
  $ sudo ./execsnoop.py
  /virtual/main.c:99:157: error: expected ')'
    data.ppid = ({ typeof(pid_t) _val; __builtin_memset(&_val, 0, sizeof(_val)); bpf_probe_read(&_val, sizeof(_val),
       (void *)&({ typeof(struct task_struct  btf_type_tag(rcu)*) _val; __builtin_memset(&_val, 0, sizeof(_val));
                                              ^
        bpf_probe_read(&_val, sizeof(_val), (void *)&task->real_parent); _val; })->tgid); _val; });

The failure reason is due to that the bcc rewriter printed type like
  struct task_struct  btf_type_tag(rcu)*
where the compiler cannot recognize what 'btf_type_tag(rcu)' is.

The above type is printed in [2] by UnaryOperator->getType().getAsString() (from clang)
in function ProbeVisitor::VisitUnaryOperator.

The original source type looks like ([3])
  struct task_struct {
    ...
    struct task_struct __rcu	*real_parent;
    ...
  }
  where '__rcu' is a macro expanding to '__attribute__((btf_type_tag("rcu")))'.

Let us print btf_type_tag properly in clang so bcc tools and broader type printing
will work properly.

With this patch, the above rewrited source code looks like
    data.ppid = ({ typeof(pid_t) _val; __builtin_memset(&_val, 0, sizeof(_val)); bpf_probe_read(&_val, sizeof(_val),
       (void *)&({ typeof(struct task_struct  __attribute__((btf_type_tag("rcu")))*) _val; __builtin_memset(&_val, 0, sizeof(_val));
        bpf_probe_read(&_val, sizeof(_val), (void *)&task->real_parent); _val; })->tgid); _val; });

and execsnoop.py tool can run properly.

  [1] https://github.com/iovisor/bcc/blob/master/tools/exitsnoop.py
  [2] https://github.com/iovisor/bcc/blob/master/src/cc/frontends/clang/b_frontend_action.cc
  [3] https://github.com/torvalds/linux/blob/master/include/linux/sched.h

Differential Revision: https://reviews.llvm.org/D150017

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