[Clang][Attr] Support btf_type_tag attribute

[Clang][Attr] Support btf_type_tag attribute

This patch introduced btf_type_tag attribute. The attribute
is a type attribute and intends to address the below
linux use cases.
    typedef int __user *__intp;
    int foo(int __user *arg, ...)
    static int do_execve(struct filename *filename,
        const char __user *const __user *__argv,
        const char __user *const __user *__envp)

Here __user in the kernel defined as
    __attribute__((noderef, address_space(__user)))
for sparse ([1]) type checking mode.

For normal clang compilation, we intend to replace it with
    __attribute__((btf_type_tag("user")))
and record such informaiton in dwarf and BTF so such
information later can be used in kernel for bpf verification
or for other tracing functionalities.

  [1] https://www.kernel.org/doc/html/v4.11/dev-tools/sparse.html

Differential Revision: https://reviews.llvm.org/D111199