BPF: Support btf_type_tag attribute

A new kind BTF_KIND_TYPE_TAG is defined. The tags associated
with a pointer type are emitted in their IR order as modifiers.
For example, for the following declaration:
  int __tag1 * __tag1 __tag2 *g;
The BTF type chain will look like
  VAR(g) -> __tag1 --> __tag2 -> pointer -> __tag1 -> pointer -> int
In the above "->" means BTF CommonType.Type which indicates
the point-to type.

Differential Revision: https://reviews.llvm.org/D113222