Serialization: Switch over to using the native SmallVector based BitstreamWriter
ctor.

llvm-svn: 151752
diff --git a/clang/lib/Frontend/ASTUnit.cpp b/clang/lib/Frontend/ASTUnit.cpp
index 3ffb6ec..c1678d4 100644
--- a/clang/lib/Frontend/ASTUnit.cpp
+++ b/clang/lib/Frontend/ASTUnit.cpp
@@ -2423,7 +2423,7 @@
   if (getDiagnostics().hasErrorOccurred())
     return true;
 
-  std::vector<unsigned char> Buffer;
+  SmallString<128> Buffer;
   llvm::BitstreamWriter Stream(Buffer);
   ASTWriter Writer(Stream);
   // FIXME: Handle modules