Remove \brief commands from doxygen comments.
This is similar to the LLVM change https://reviews.llvm.org/D46290.
We've been running doxygen with the autobrief option for a couple of
years now. This makes the \brief markers into our comments
redundant. Since they are a visual distraction and we don't want to
encourage more \brief markers in new code either, this patch removes
Patch produced by
for i in $(git grep -l '\@brief'); do perl -pi -e 's/\@brief g' $i & done
for i in $(git grep -l '\\brief'); do perl -pi -e 's/\\brief g' $i & done
Differential Revision: https://reviews.llvm.org/D46320