This commit adds infrastructure for conversion and testing it.
The conversion is still incomplete, pending discussion about which tags should
we use in the XML documents. I copied the RelaxNG schema from Clang, and will
edit it accordingly.
Swift SVN r16451
It does not yet understand a lot of ReST constructs, in particular, it does not
understand inline markup at all, but it is good enough to extract brief
comments.
The parser in BriefParser.cpp does not construct any ASTs or emit any warnings.
This is a special parser for brief comments that should be as fast as possible.
Real ReST parser will come later.
Swift SVN r15046
