[LLVM][Support] Fix tests on Cygwin

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 on Cygwin as it
relies on this behavior.

Author: jeremyd2019

llvm/unittests/Support/CommandLineTest.cpp

@@ -28,6 +28,9 @@
 #include <fstream>
 #include <stdlib.h>
 #include <string>
+#if HAVE_UNISTD_H
+#include <unistd.h>
+#endif
 
 using namespace llvm;
 using llvm::unittest::TempDir;
@@ -834,14 +837,23 @@ TEST(CommandLineTest, DefaultOptions) {
 }
 
 TEST(CommandLineTest, ArgumentLimit) {
-  std::string args(32 * 4096, 'a');
-  EXPECT_FALSE(llvm::sys::commandLineFitsWithinSystemLimits("cl", args.data()));
+#if HAVE_UNISTD_H && defined(_SC_ARG_MAX)
+  if (sysconf(_SC_ARG_MAX) != -1) {
+#endif
+    std::string args(32 * 4096, 'a');
+    EXPECT_FALSE(
+        llvm::sys::commandLineFitsWithinSystemLimits("cl", args.data()));
+#if HAVE_UNISTD_H && defined(_SC_ARG_MAX)
+  }
+#endif
   std::string args2(256, 'a');
   EXPECT_TRUE(llvm::sys::commandLineFitsWithinSystemLimits("cl", args2.data()));
 }
 
 TEST(CommandLineTest, ArgumentLimitWindows) {
-  if (!Triple(sys::getProcessTriple()).isOSWindows())
+  Triple processTriple(sys::getProcessTriple());
+  if (!processTriple.isOSWindows() ||
+      processTriple.isWindowsCygwinEnvironment())
     GTEST_SKIP();
   // We use 32000 as a limit for command line length. Program name ('cl'),
   // separating spaces and termination null character occupy 5 symbols.
@@ -854,7 +866,9 @@ TEST(CommandLineTest, ArgumentLimitWindows) {
 }
 
 TEST(CommandLineTest, ResponseFileWindows) {
-  if (!Triple(sys::getProcessTriple()).isOSWindows())
+  Triple processTriple(sys::getProcessTriple());
+  if (!processTriple.isOSWindows() ||
+      processTriple.isWindowsCygwinEnvironment())
     GTEST_SKIP();
 
   StackOption<std::string, cl::list<std::string>> InputFilenames(

llvm/unittests/Support/DynamicLibrary/PipSqueak.h

@@ -22,7 +22,7 @@
 #include <vector>
 #endif
 
-#ifdef _WIN32
+#if defined(_WIN32) || defined(__CYGWIN__)
 #define PIPSQUEAK_EXPORT __declspec(dllexport)
 #elif defined(__MVS__)
 #define PIPSQUEAK_EXPORT __attribute__((__visibility__("default")))

llvm/unittests/Support/VirtualFileSystemTest.cpp

@@ -558,6 +558,10 @@ 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();