Remove dead code

[• rafael]

Summary

I think this code is dead since rename was reimplemented on windows in r315079.

[davide]

Seems safe (assuming it doesn't break windows)

[pcc]

Unfortunately, we were still seeing occasional permission denied errors on our bots even with that change.
e.g. https://build.chromium.org/p/chromium.clang/builders/ToTWinThinLTO64/builds/101
So this code was added as part of D39874.

The confusion is understandable. Bob, can you please update the comment to make it clear why we need that code even with a better implementation of rename?

lib/LTO/Caching.cpp

struct CacheStream : NativeObjectStream {
         // Open the file first to avoid racing with a cache pruner.
         ErrorOr<std::unique_ptr<MemoryBuffer>> MBOrErr =
             MemoryBuffer::getFile(TempFilename);
         if (!MBOrErr)
           report_fatal_error(Twine("Failed to open new cache file ") +
                              TempFilename + ": " +
                              MBOrErr.getError().message() + "\n");
 
-        // This is atomic on POSIX systems.
-        // On Windows, it can fail with permission denied if the destination
-        // file already exists. Since the existing file should be semantically
-        // equivalent to the one we are trying to write, we give AddBuffer
-        // a copy of the bytes we wrote in that case. We do this instead of
-        // just using the existing file, because the pruner might delete the
-        // file before we get a chance to use it.
-        auto EC = sys::fs::rename(TempFilename, EntryPath);
-        if (EC == errc::permission_denied) {
-          auto MBCopy = MemoryBuffer::getMemBufferCopy(
-              (*MBOrErr)->getBuffer(), EntryPath);
-          MBOrErr = std::move(MBCopy);
-          sys::fs::remove(TempFilename);
-        } else if (EC) {
+        if (auto EC = sys::fs::rename(TempFilename, EntryPath)) {
           report_fatal_error(Twine("Failed to rename temporary file ") +
                              TempFilename + " to " + EntryPath + ": " +
                              EC.message() + "\n");
         }
 
         AddBuffer(Task, std::move(*MBOrErr), EntryPath);
       }
     };