[clang] [clang][Dependency Scanning] Canonicalize Defines of a Compiler Invocation As Early As Possible (PR #159620)

[Qiongsi Wu via cfe-commits]

qiongsiwu wrote:

> > Note to reviewers:
> > I did not add any tests here since I don't think there are visible behavior change observable without CAS. I am testing this work using https://github.com/swiftlang/swift and its llvm fork. Please let me know if there are things I can test against and I will add a test.
> > Thanks!
> 
> Do we have test downstream? Can you link a PR with test added?

Here's a downstream test https://github.com/swiftlang/llvm-project/pull/11454. 

https://github.com/llvm/llvm-project/pull/159620