[Preamble] Fix an undefined behavior when checking an empty preamble can be reused.

[hokein]

Summary

Passing nullptr to memcmp is UB.

[hokein]

Use the std::equal to replace the memcmp.

[ilya-biryukov]

LGTM. Thanks!

cfe/trunk/lib/Frontend/PrecompiledPreamble.cpp

PreprocessorOptions &PreprocessorOpts =
       PreambleInvocation->getPreprocessorOpts();
 
   // We've previously computed a preamble. Check whether we have the same
   // preamble now that we did before, and that there's enough space in
   // the main-file buffer within the precompiled preamble to fit the
   // new main file.
   if (PreambleBytes.size() != Bounds.Size ||
       PreambleEndsAtStartOfLine != Bounds.PreambleEndsAtStartOfLine ||
-      memcmp(PreambleBytes.data(), MainFileBuffer->getBufferStart(),
-             Bounds.Size) != 0)
+      !std::equal(PreambleBytes.begin(), PreambleBytes.end(),
+                  MainFileBuffer->getBuffer().begin()))
     return false;
   // The preamble has not changed. We may be able to re-use the precompiled
   // preamble.
 
   // Check that none of the files used by the preamble have changed.
   // First, make a record of those files that have been overridden via
   // remapping or unsaved_files.
   std::map<llvm::sys::fs::UniqueID, PreambleFileHash> OverriddenFiles;