clang-format: [JS] handle default bindings in imports.

Summary:
Default imports appear outside of named bindings in curly braces:

  import A from 'a';
  import A, {symbol} from 'a';

Reviewers: djasper

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D23973

llvm-svn: 280486