[clang-doc] Always emit the TagType for RecordInfo

Always emit the TagType for RecordInfo in YAML output. Previously this omitted the type for "struct", considering it the default. But records in C++ don't really have a default type so always emitting this is more clear.

Emit IsTypeDef in YAML. Previously this existed only in the Representation but was never written. Additionally, adds IsTypeDef to the record merge operation which was clearing it (all RecordInfo structures are merged with am empty RecordInfo during the reduce phase).

Reviewed By: paulkirth

Differential Revision: https://reviews.llvm.org/D131739