[clang-doc] Serialize child namespaces and records

Serialization of child namespaces and records is now handled.
Namespaces can have child records and child namespaces.
Records can only have child records.

Committed on behalf of Diego Astiazarán (diegoaat97@gmail.com).

Differential Revision: https://reviews.llvm.org/D63911

llvm-svn: 364963