Escape ]]> in xunit xml output

Summary:
This sequence ends the CDATA block so any characters after that are no
longer escaped. This can be fixed by replacing "]]>" with "]]]]><![CDATA[>".

Reviewers: cmatthews

Reviewed By: cmatthews

Differential Revision: https://reviews.llvm.org/D46886

llvm-svn: 332440