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

[Nico Weber via cfe-commits]

nico wrote:

Also failing here https://lab.llvm.org/buildbot/#/builders/259/builds/126 and here https://lab.llvm.org/buildbot/#/builders/139/builds/59858

Given that the bots have been broken for a few hours now, I'll revert.

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