clang-format: [JS] test declared fields.

Summary:
TypeScript now supports declaring fields:

    class Foo {
      declare field: string;
    }

clang-format happens to already format this fine, so this change just
adds a regression test.

Reviewers: krasimir

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69972