clang-format: [JS] Sort imported symbols.

Summary: E.g. sort `import {b, a} from 'x';` into `import {a, b} from 'x';`.

Reviewers: djasper

Subscribers: cfe-commits, klimek

Differential Revision: http://reviews.llvm.org/D20798

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@271400 91177308-0d34-0410-b5e6-96231b3b80d8