[AST] Refactor propagation of dependency bits. NFC

Summary:
This changes introduces an enum to represent dependencies as a bitmask
and extract common patterns from code that computes dependency bits into
helper functions.

Reviewers: rsmith, martong, shafik, ilya-biryukov, hokein

Subscribers: hokein, sammccall, Mordante, riccibruno, merge_guards_bot, rnkovacs, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D71920