Fix buffer-overflow in llvm-mt's notify_update feature.

[hctim]

Summary

The 3-parameter std::equal used in this code access FileBuffer from [0,
OutputBuffer->getBufferEnd() - OutputBuffer->getBufferStart()). If the
size of FileBuffer is shorter than OutputBuffer, this ends up
overflowing.

This wasn't found on the sanitizer buildbots as they use an instrumented
libcxx, and libcxx implements std::equal using a loop. libstdc++ on my
local macine finds the bug, as it implements std::equal using bcmp(),
which ASan intercepts and does a range check.

The existing test doesn't technically do a buffer-overflow, but the code
definitely can. If OutputBuffer was "AAABBB" and FileBuffer was "AAA",
then the code would overflow.

[abrachet]

Thanks for catching it, there is already D139315 which should address this too. But this is good to go if you want to submit it now

llvm/tools/llvm-mt/llvm-mt.cpp

int llvm_mt_main(int Argc, char **Argv) {
   if (InputArgs.hasArg(OPT_notify_update)) {
     ErrorOr<std::unique_ptr<MemoryBuffer>> OutBuffOrErr =
         MemoryBuffer::getFile(OutputFile);
     // Assume if we couldn't open the output file then it doesn't exist meaning
     // there was a change.
     bool Same = false;
     if (OutBuffOrErr) {
       const std::unique_ptr<MemoryBuffer> &FileBuffer = *OutBuffOrErr;
-      Same = std::equal(OutputBuffer->getBufferStart(),
-                        OutputBuffer->getBufferEnd(),
-                        FileBuffer->getBufferStart());
+      Same = std::equal(
+          OutputBuffer->getBufferStart(), OutputBuffer->getBufferEnd(),
+          FileBuffer->getBufferStart(), FileBuffer->getBufferEnd());
     }
     if (!Same) {
 #if LLVM_ON_UNIX
       ExitCode = 0xbb;
 #elif defined(_WIN32)
       ExitCode = 0x41020001;
 #endif
     }