reland: [clang][ScanDeps] Canonicalize -D and -U flags (#82568)

Canonicalize `-D` and `-U` flags by sorting them and only keeping the
last instance of a given name.

This optimization will only fire if all `-D` and `-U` flags start with a
simple identifier that we can guarantee a simple analysis of can
determine if two flags refer to the same identifier or not. See the
comment on `getSimpleMacroName()` for details of what the issues are.

Previous version of this had issues with sed differences between macOS,
Linux, and Windows. This test doesn't check paths, so just don't run
sed.
Other tests should use `sed -E 's:\\\\?:/:g'` to get portable behavior.

Windows has different command line parsing behavior than Linux for
compilation databases, so the test has been adjusted to ignore that
difference.