libclang API for comment-to-xml conversion.

The implementation also includes a Relax NG schema and tests for the schema
itself.  The schema is used in c-index-test to verify that XML documents we
produce are valid.  In order to do the validation, we add an optional libxml2
dependency for c-index-test.

Credits for CMake part go to Doug Gregor.  Credits for Autoconf part go to Eric
Christopher.  Thanks!

llvm-svn: 161431