[Serialization] Ensure that a cached bridging header is null-terminated.

I'm not quite sure how to tickle this one, but the next commit adds more
data after the cached header, at which point existing tests break. This
could have already caused problems if no padding was needed in the bitstream.

Swift SVN r21543

lib/Serialization/Serialization.cpp

@@ -484,8 +484,10 @@ void Serializer::writeInputFiles(FilenamesTy inputFiles,
       ImportedHeader.emit(ScratchRecord, publicImportSet.count(import),
                           importedHeaderSize, importedHeaderModTime,
                           importedHeader);
-      if (!contents.empty())
+      if (!contents.empty()) {
+        contents.push_back('\0');
         ImportedHeaderContents.emit(ScratchRecord, contents);
+      }
       continue;
     }