[clang-doc] Setup clang-doc frontend framework

Setting up the mapper part of the frontend framework for a clang-doc
tool. It creates a series of relevant matchers for declarations, and
uses the ToolExecutor to traverse the AST and extract the matching
declarations and comments. The mapper serializes the extracted
information to individual records for reducing and eventually doc
generation.

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

Differential Revision: https://reviews.llvm.org/D41102

llvm-svn: 327102