Implement AST classes for comments, a real parser for Doxygen comments and a

Implement AST classes for comments, a real parser for Doxygen comments and a
very simple semantic analysis that just builds the AST; minor changes for lexer
to pick up source locations I didn't think about before.

Comments AST is modelled along the ideas of HTML AST: block and inline content.

* Block content is a paragraph or a command that has a paragraph as an argument
  or verbatim command.
* Inline content is placed within some block.  Inline content includes plain
  text, inline commands and HTML as tag soup.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@159790 91177308-0d34-0410-b5e6-96231b3b80d8