[clang-doc] Fix records in global namespace

When a Record is declared in the global namespace, clang-doc serializes
it as a child of the global namespace, so the global namespace is now
one if its parent namespaces. This namespace was not being included in
the list of namespaces of the Info causing paths to be incorrect and the
index rendered incorrectly.
Affected tests have been fixed.

Differential revision: https://reviews.llvm.org/D66298

llvm-svn: 369123