[cleanup] Switch comments to use '\brief' style instead of '@brief'

[cleanup] Switch comments to use '\brief' style instead of '@brief'
style, and remove some unnecessary comments (the code is perfectly
self-documenting here). Also clang-format the function declarations as
they wrap cleanly now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199084 91177308-0d34-0410-b5e6-96231b3b80d8