clang-format: [JS] do not collapse short classes.

Summary:
clang-format does not collapse short records, interfaces, unions, etc.,
but fails to do so if the record is preceded by certain modifiers
(export, default, abstract, declare). This change skips over all
modifiers, and thus handles all record definitions uniformly.

Before:
    export class Foo { bar: string; }
    class Baz {
      bam: string;
    }

After:
    export class Foo {
      bar: string;
    }
    class Baz {
      bam: string;
    }

Reviewers: djasper

Subscribers: klimek

Differential Revision: https://reviews.llvm.org/D40430

llvm-svn: 318976