[clang] [clang][Dependency Scanning][NFC] Move `DependencyScanningAction` to its own header and source files (PR #160795)

[Qiongsi Wu via cfe-commits]

qiongsiwu wrote:

Thanks so much for the prompt review! The latest commit:

1. Reordered the private/helper functions to keep the original order as much as possible (and processed with clang-format). `sanitizeDiagOpts` is the outstanding one since it is used in two different files and I kept its location in the file.
2. The new files are renamed and new lines are added to the ends. 

https://github.com/llvm/llvm-project/pull/160795