Implement a (simple) Markdown generator

Implementing a simple Markdown generator from the emitted bitcode
summary of declarations. Very primitive at this point, but will be
expanded. Currently emits an .md file for each class and namespace,
listing its contents.

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/D43424

llvm-svn: 339948