Remove llvm::MemoryBuffer const_casts

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.

Reviewers: dblaikie, rsmith

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D41387

llvm-svn: 321167
diff --git a/clang/lib/Lex/Preprocessor.cpp b/clang/lib/Lex/Preprocessor.cpp
index c291a4b9..7d789e7 100644
--- a/clang/lib/Lex/Preprocessor.cpp
+++ b/clang/lib/Lex/Preprocessor.cpp
@@ -420,10 +420,9 @@
   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);