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!


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@161431 91177308-0d34-0410-b5e6-96231b3b80d8