[clang-doc] Generate HTML links for children namespaces/records

Path is now stored in the references to the child while serializing,
then this path is used to generate the relative path in the HTML
generator.
Now some references have paths and some don't so in the reducing phase,
references are now properly merged checking for empty attributes.
Tests added for HTML and YAML generators, merging and serializing.
computeRelativePath function had a bug when the filepath is part of the
given directory; it returned a path that starts with a separator. This
has been fixed.

Differential Revision: https://reviews.llvm.org/D65987

llvm-svn: 368602