Enable AUTOBRIEF in doxygen configuration.

This brings the LLDB configuration closer to LLVM's and removes visual
clutter in the source code by removing the @brief commands from
comments.

This patch also reflows the paragraphs in all doxygen comments.

See also https://reviews.llvm.org/D46290.

Differential Revision: https://reviews.llvm.org/D46321

llvm-svn: 331373