clang-format: [JS] handle parenthesized class expressions.

Summary:
In JavaScript, classes are expressions, so they can appear e.g. in
argument lists.

    var C = foo(class {
      bar() {
        return 1;
      }
    };

Reviewers: djasper

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D29635

llvm-svn: 294302