[Path] Set FD to -1 in moved-from TempFile

[JDevlieghere]

Summary

When moving a temp file, explicitly set the file descriptor to -1 so we can never accidentally close the moved-from TempFile.

[shafik]

Thanks for fixing this! LGTM

llvm/lib/Support/Path.cpp

 namespace sys {
 namespace fs {
 TempFile::TempFile(StringRef Name, int FD) : TmpName(Name), FD(FD) {}
 TempFile::TempFile(TempFile &&Other) { *this = std::move(Other); }
 TempFile &TempFile::operator=(TempFile &&Other) {
   TmpName = std::move(Other.TmpName);
   FD = Other.FD;
   Other.Done = true;
+  Other.FD = -1;
   return *this;
 }
 
 TempFile::~TempFile() { assert(Done); }
 
 Error TempFile::discard() {
   Done = true;
   if (FD != -1 && close(FD) == -1) {

llvm/unittests/Support/Path.cpp

 #endif
 
 
 TEST_F(FileSystemTest, TempFileKeepDiscard) {
   // We can keep then discard.
   auto TempFileOrError = fs::TempFile::create(TestDirectory + "/test-%%%%");
   ASSERT_TRUE((bool)TempFileOrError);
   fs::TempFile File = std::move(*TempFileOrError);
+  ASSERT_EQ(-1, TempFileOrError->FD);
   ASSERT_FALSE((bool)File.keep(TestDirectory + "/keep"));
   ASSERT_FALSE((bool)File.discard());
   ASSERT_TRUE(fs::exists(TestDirectory + "/keep"));
   ASSERT_NO_ERROR(fs::remove(TestDirectory + "/keep"));
 }
 
 TEST_F(FileSystemTest, TempFileDiscardDiscard) {
   // We can discard twice.
   auto TempFileOrError = fs::TempFile::create(TestDirectory + "/test-%%%%");
   ASSERT_TRUE((bool)TempFileOrError);
   fs::TempFile File = std::move(*TempFileOrError);
+  ASSERT_EQ(-1, TempFileOrError->FD);
   ASSERT_FALSE((bool)File.discard());
   ASSERT_FALSE((bool)File.discard());
   ASSERT_FALSE(fs::exists(TestDirectory + "/keep"));
 }
 
 TEST_F(FileSystemTest, TempFiles) {
   // Create a temp file.
   int FileDescriptor;