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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@151752 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Frontend/ASTUnit.cpp b/lib/Frontend/ASTUnit.cpp
index 3ffb6ec..c1678d4 100644
--- a/lib/Frontend/ASTUnit.cpp
+++ b/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