[clang-scan-deps] Enable test P1689.cppm on Windows (#145857)
The test `test/ClangScanDeps/P1689.cppm` was previously not supported on
Windows due to the differences in path separators between Windows and
Linux. This normalizes the paths, allowing this test to run on Windows
as well.
This is motivated by the review feedback for #145221, where the same
change was suggested for a test derived from this one.