[LLVM][Support] Fix tests on Cygwin (#151417)

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.

1 files changed, 4 insertions, 0 deletions

diff --git a/llvm/unittests/Support/VirtualFileSystemTest.cpp b/llvm/unittests/Support/VirtualFileSystemTest.cpp
index fc3ccea..6228de8 100644
--- a/llvm/unittests/Support/VirtualFileSystemTest.cpp
+++ b/llvm/unittests/Support/VirtualFileSystemTest.cpp
@@ -553,6 +553,10 @@ TEST(VirtualFileSystemTest, PhysicalFileSystemWorkingDirFailure) {
     // Some platforms (e.g. Solaris) disallow removal of the working directory.
     GTEST_SKIP() << "test requires deletion of working directory";
 
+#ifdef __CYGWIN__
+  GTEST_SKIP() << "Cygwin getcwd succeeds with unlinked working directory";
+#endif
+
   // Verify that we still get two separate working directories.
   auto FS1 = vfs::createPhysicalFileSystem();
   auto FS2 = vfs::createPhysicalFileSystem();