clang-format: Understand __attribute__s preceding parameter lists.

Before:
  ReturnType __attribute__((unused))
      function(int i);

After:
  ReturnType __attribute__((unused))
  function(int i);

This fixes llvm.org/PR18632.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@200337 91177308-0d34-0410-b5e6-96231b3b80d8