clang-format: [JS] Support AllowShortFunctionsOnASingleLine.

Specifically, this also counts for stuff like (with style "inline"):
  var x = function() {
    return 1;
  };

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218689 91177308-0d34-0410-b5e6-96231b3b80d8