[clang-doc] Limit integration tests

Now that the clang-doc libraries are covered by unit tests, we don't
need to have extensive (and unmaintainable) integration tests. This
replaces the integration test suite with a smaller one that just tests
the tool itself and removes extraneous dumping logic from the tool
itself.

Includes tests that cover the parse->serialize->merge->generate
pipeline, as well as tests for the --public, --format, --doxygen, and
--output flags.

Differential Revision: https://reviews.llvm.org/D53150

llvm-svn: 344655