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

[Qiongsi Wu via cfe-commits]

qiongsiwu wrote:

> LGTM, thanks! FWIW I think this is easy to test even upstream by scanning two TUs with different defines that canonicalize to the same thing, and then checking the number of entries in the module cache. Might be worth doing here.

Ah great suggestion! Thanks! A test is now added here as well. 

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