Reland "[clang-doc] Refactoring mapper to map by scope"

Relanding with a minor change to prevent an assertion on release bots.

The result of this adjusted mapper pass is that all Function and Enum
infos are absorbed into the info of their enclosing scope (i.e. the class
or namespace in which they are defined). Namespace and Record infos are
passed along to the final output, but the second pass creates a reference
to each in its parent scope. As a result, the top-level final outputs are
Namespaces and Records.

Differential Revision: https://reviews.llvm.org/D48341

llvm-svn: 338763