clang-format: Add support for a space after @property

Mozilla and WebKit seem to use a space after @property (verified by
grepping their codebases) so we turn this on there as well.

Change by Christian Legnitto. Thank you!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@200320 91177308-0d34-0410-b5e6-96231b3b80d8