llvm-mt: Fix release of OutputDoc

Summary:
xmlDoc needs to be released with xmlFreeDoc.
Reset root element before release to avoid release of CombinedRoot owned
by CombinedDoc,

Reviewers: ecbeckmann, rnk, zturner, ruiu

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D37321

llvm-svn: 312207