[clang] [clang][ScanDeps] Canonicalize -D and -U flags (PR #82298)

[Nico Weber via cfe-commits]

nico wrote:

The test is failing at least on my bot: http://45.33.8.238/linux/131314/step_7.txt

I'm guessing this is some unicode/sed thing? It's a pretty vanilla linux machine.

(It uses a non-standard non-supported build system, but at least from a distance that looks unrelated?)

https://github.com/llvm/llvm-project/pull/82298