[AST] Make Expr::setDependence protected and remove add/removeDependence. NFC

Summary:
The expected pattern is for subclasses to initialize through
computeDependence, which needs only setDependence.
The few places that still use addDependence can be simulated with get+set.

Reviewers: hokein

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D76392