[clang] Canonicalize clang-scan-deps input-file/file-deps paths for Windows (PR #155908)

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

Is the motivation mainly testing? If so, I'd suggest either hiding this behavior behind a flag, and then enabling the flag in tests and keeping the paths unmodified by default in production. Another way of doing test checks in a cross-platform way is the `sed 's:\\\\\?:/:g'` dance on the scanner output that we do in `clang/test/ClangScanDeps`.

https://github.com/llvm/llvm-project/pull/155908