Misc cleanups to the FileSytem api.

The main difference is the removal of

std::error_code exists(const Twine &path, bool &result);

It was an horribly redundant interface since a file not existing is also a valid
error_code. Now we have an access function that returns just an error_code. This
is the only function that has to be implemented for Unix and Windows. The
functions can_write, exists and can_execute an now just wrappers.

One still has to be very careful using these function to avoid introducing
race conditions (Time of check to time of use).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217625 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/LockFileManager.cpp

@@ -204,8 +204,8 @@ LockFileManager::WaitForUnlockResult LockFileManager::waitForUnlock() {
     // If the lock file is still expected to be there, check whether it still
     // is.
     if (!LockFileGone) {
-      bool Exists;
-      if (!sys::fs::exists(LockFileName.str(), Exists) && !Exists) {
+      if (sys::fs::access(LockFileName.c_str(), sys::fs::AccessMode::Exist) ==
+          errc::no_such_file_or_directory) {
         LockFileGone = true;
         LockFileJustDisappeared = true;
       }