[clang-doc] Read docstrings for record members

[clang-doc] Read docstrings for record members

Struct/class data members did not have the comments associated with
them. This adds that information to the MemberTypeInfo class and emits
it in the YAML. This does not update the frontends yet.

Reviewed By: paulkirth

Differential Revision: https://reviews.llvm.org/D131298