`CompilationDatabaseTests` have been flakey on Windows since
https://github.com/swiftlang/sourcekit-lsp/pull/2334. This didn't really
introduce the failure though - it just added tests that now catch it.
The underlying cause is that `filename.isAbsolutePath` sometimes returns
`true` for a relative path, which then causes the CWD to be prepended
instead of the given `compileCommandsDirectory`.
`PathIsRelativeW` requires a null terminated string, so presumably we
were hitting junk after the initial path that sometimes turned it into
an absolute path. From the Windows docs:
> A pointer to a null-terminated string of maximum length MAX_PATH that
contains the path to search.
Fixes#2360
Resolves rdar://165006835