[clang][tools] Remove path separator assumption

After 98e6deb6 the 'HeadersForSymbolTest.IWYUTransitiveExportWithPrivate' test in 'ClangIncludeCleanerTest' started failing. This is most likely because `FileEntryRef::getName()` now starts with ".\" on Windows, whereas `FileEntry::getName()` did not. This commit fixes assumption of forward slash separators.