Remove llvm::MemoryBuffer const_casts

[labath]

Summary

llvm has grown a WritableMemoryBuffer class, which is convertible
(inherits from) a MemoryBuffer. We can use it to avoid conts_casting the
buffer contents when we want to write to it.

[dblaikie]

Seems good

lib/Basic/SourceManager.cpp

llvm::MemoryBuffer *ContentCache::getBuffer(DiagnosticsEngine &Diag,
   // really deal with this situation, just create an empty buffer.
   //
   // FIXME: This is definitely not ideal, but our immediate clients can't
   // currently handle returning a null entry here. Ideally we should detect
   // that we are in an inconsistent situation and error out as quickly as
   // possible.
   if (!BufferOrError) {
     StringRef FillStr("<<<MISSING SOURCE FILE>>>\n");
-    Buffer.setPointer(MemoryBuffer::getNewUninitMemBuffer(
-                          ContentsEntry->getSize(), "<invalid>").release());
-    char *Ptr = const_cast<char*>(Buffer.getPointer()->getBufferStart());
+    auto BackupBuffer = llvm::WritableMemoryBuffer::getNewUninitMemBuffer(
+        ContentsEntry->getSize(), "<invalid>");
+    char *Ptr = BackupBuffer->getBufferStart();
     for (unsigned i = 0, e = ContentsEntry->getSize(); i != e; ++i)
       Ptr[i] = FillStr[i % FillStr.size()];
+    Buffer.setPointer(BackupBuffer.release());
 
     if (Diag.isDiagnosticInFlight())
       Diag.SetDelayedDiagnostic(diag::err_cannot_open_file,
                                 ContentsEntry->getName(),
                                 BufferOrError.getError().message());
     else
       Diag.Report(Loc, diag::err_cannot_open_file)
           << ContentsEntry->getName() << BufferOrError.getError().message();

lib/Lex/Preprocessor.cpp

bool Preprocessor::SetCodeCompletionPoint(const FileEntry *File,
   }
 
   if (Position > Buffer->getBufferEnd())
     Position = Buffer->getBufferEnd();
 
   CodeCompletionFile = File;
   CodeCompletionOffset = Position - Buffer->getBufferStart();
 
-  std::unique_ptr<MemoryBuffer> NewBuffer =
-      MemoryBuffer::getNewUninitMemBuffer(Buffer->getBufferSize() + 1,
-                                          Buffer->getBufferIdentifier());
-  char *NewBuf = const_cast<char*>(NewBuffer->getBufferStart());
+  auto NewBuffer = llvm::WritableMemoryBuffer::getNewUninitMemBuffer(
+      Buffer->getBufferSize() + 1, Buffer->getBufferIdentifier());
+  char *NewBuf = NewBuffer->getBufferStart();
   char *NewPos = std::copy(Buffer->getBufferStart(), Position, NewBuf);
   *NewPos = '\0';
   std::copy(Position, Buffer->getBufferEnd(), NewPos+1);
   SourceMgr.overrideFileContents(File, std::move(NewBuffer));
 
   return false;
 }