Fix msan buildbot.

This patch should fix sanitizer-x86_64-linux-fast bot.

The problem was that the contents of this stream are aligned to 4 byte,
and the paddings were created just by incrementing `Offset`, so paddings
had undefined values. When the entire stream is written to an output,
it triggered msan.

llvm-svn: 305541

lib/Support/BinaryStreamWriter.cpp

@@ -83,6 +83,7 @@ Error BinaryStreamWriter::padToAlignment(uint32_t Align) {
   uint32_t NewOffset = alignTo(Offset, Align);
   if (NewOffset > getLength())
     return make_error<BinaryStreamError>(stream_error_code::stream_too_short);
-  Offset = NewOffset;
+  while (Offset < NewOffset)
+    writeInteger('\0');
   return Error::success();
 }