Cygwin returns -1 for `getconf(_SC_ARG_MAX)`, which makes
`llvm::sys::commandLineFitsWithinSystemLimits` always return true, so
skip the `ArgumentLimit` test in that case. Skip the
`ArgumentLimitWindows` and `ResponseFileWindows` tests on Cygwin also as
it doesn't suffer from the Windows limits either.
Cygwin requires the same `dllexport` annotation as Win32 in the
`DynamicLibrary` test, so add its preprocessor check to PipSqueak.h.
Cygwin's `getcwd` function does not fail with `ENOENT` if the current
working directory is unlinked. According to POSIX issue 8, this is not
required. Skip the `PhysicalFileSystemWorkingDirFailure` test on Cygwin
as it relies on this behavior.
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