[AST] Add a Dependence bitmask to use for calculations with multiple node types.

Summary:
This makes it easier/safer to add bits (error) to other node types without
worrying about bit layout all the time.

For now, just use to implement the ad-hoc conversion functions.
Next: remove these functions and use this directly.

Reviewers: hokein

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D76939