[Includecleaner] Introduce RefType to ast walking

RefTypes are distinct categories for each reference to a symbol. They
are signals indicating strength of a reference, that'll enable different
decision making based on the finding being provided.

There are 3 kinds of ref types:
- Explicit, the reference is spelled in the code.
- Implicit, the reference is not directly visible in the code.
- Ambigious, the reference exists but can't be proven as used (e.g.
  overloads brought in by a using decl but not used by the code).

Differential Revision: https://reviews.llvm.org/D135859