[clang-doc] Implement a YAML generator

Implmenting a YAML generator from the emitted bitcode summary of
declarations. Emits one YAML file for each declaration information.

For a more detailed overview of the tool, see the design document on the mailing list: http://lists.llvm.org/pipermail/cfe-dev/2017-December/056203.html

llvm-svn: 334103