swift234

Implement some elements of a ReST parser

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