Const-correctness in libLTO

Looks like I was sloppy when bridging to C.
Thanks D. Blaikie for noticing!

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 263885
diff --git a/llvm/tools/lto/lto.cpp b/llvm/tools/lto/lto.cpp
index 3af8f7d..165b23d 100644
--- a/llvm/tools/lto/lto.cpp
+++ b/llvm/tools/lto/lto.cpp
@@ -469,7 +469,7 @@
                                           unsigned int index) {
   assert(index < unwrap(cg)->getProducedBinaries().size() && "Index overflow");
   auto &MemBuffer = unwrap(cg)->getProducedBinaries()[index];
-  return LTOObjectBuffer{const_cast<char *>(MemBuffer->getBufferStart()),
+  return LTOObjectBuffer{MemBuffer->getBufferStart(),
                          MemBuffer->getBufferSize()};
 }