[Tooling/DependencyScanning] Rename refactorings towards transitioning...

[Tooling/DependencyScanning] Rename refactorings towards transitioning dependency scanning to use pre-lexed preprocessor directive tokens

This is first of a series of patches for making the special lexing for dependency scanning a first-class feature of the `Preprocessor` and `Lexer`.
This patch only includes NFC renaming changes to make reviewing of the functionality changing parts easier.

Differential Revision: https://reviews.llvm.org/D125484