Fix Casting

Make the const_cast explicit to silence a compiler warning.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172560 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Serialization/ASTWriter.cpp b/lib/Serialization/ASTWriter.cpp
index d4b0367..af736a4 100644
--- a/lib/Serialization/ASTWriter.cpp
+++ b/lib/Serialization/ASTWriter.cpp
@@ -1552,7 +1552,7 @@
   
   // Free all of the strings we had to duplicate.
   for (unsigned I = 0, N = SavedStrings.size(); I != N; ++I)
-    free((void*)SavedStrings[I]);
+    free(const_cast<char *>(SavedStrings[I]));
 }
 
 /// \brief Writes the block containing the serialized form of the