Fix many doxygen formatting errors.

This patch affects docs only, and includes formatting changes only
(though those include some fixes for broken Doxygen markup that caused
some content to be missing from generated pages).  It avoids generating
many spurious pages such as
http://clang.llvm.org/doxygen/classRepresents.html, but likely not all
yet.

Patch by James Dennett.

llvm-svn: 158155