clang/test/Index/headerfile-comment-to-html.m: Try to unbreak libxml2-less hosts to eliminate "CommentXMLValid".

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174124 91177308-0d34-0410-b5e6-96231b3b80d8
Consistently format sample diagnostics on this page.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174126 91177308-0d34-0410-b5e6-96231b3b80d8