llvm-mt: Fix memory management in WindowsManifestMergerImpl::getMergedManifest

Summary:
xmlDoc needs to be released with xmlFreeDoc.
XML_PARSE_NODICT is needed for safe moving nodes between documents.
Buffer returned from xmlDocDumpFormatMemoryEnc needs xmlFree, but it needs
outlive users of getMergedManifest results.

Reviewers: ecbeckmann, rnk, zturner, ruiu

Subscribers: llvm-commits

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

llvm-svn: 312406